variations in shear wave splitting in young pacific seafloor 1 analysis of sks phases recorded by ocean bottom seismometers in the glimpse experiment has expanded the region of in situ measurements of shear wave splitting in young seafloor near the southern east pacific rise average splitting times at individual glimpse sites range from 1 1 to 2 2 s with fast direction azimuths trending approximately in the direction of absolute plate motion and fossil spreading for the pacific plate the glimpse study area at 12 to 14 s is adjacent to the melt experiment the new observations demonstrate that splitting delays are not uniform on the pacific plate and that the contrast between the nazca and pacific plates is not a fundamental consequence of the asymmetric plate motion the variations in degree of splitting could be caused by small scale convection variations in water content or rate of deformation in the asthenospheric return flow to the east pacific rise

comparative studies of organic matter petrography of the late palaeozoic black shales from southwestern poland abstract there are at least two sapropelic units associated with late palaeozoic black shales in central europe the older unit of late carboniferous age is the lower part of the anthracosia shales in the intrasudetic basin and the younger one is the well known zechstein kupferschiefer in both the foresudetic monocline and the northsudetic basin the first unit is of lacustrine origin while the second one represents deposition in a shallow marine depositional environment both units contain high amounts of organic matter thus being typical black shales the organic matter dispersed in these shales was studied petrographically in general the vitrinite reflectance of the shales studied indicates variable but moderate organic matter maturity 0 68 1 25 equivalent to the oil window detailed microscopic studies of the organic material dispersed in the lower unit of the anthracosia shales showed that liptinite especially alginite is the most abundant component secondary altered organic matter i e solid hydrocarbons rarely occurs organic components together with mineral matter constitute a lacustrine sapropelic association a humic terrestrial association and an intermediary association the character and predominance of alginite and lacustrine sapropelic association are indicative of an open lacustrine depositional environment in general this organic composition is typical of type i kerogen microscopic analysis of the kupferschiefer revealed a mixture of liptinite vitrinite and inertinite macerals and other organic components such as amorphous sapropelic mass asm and solid bitumens the most common organic components are liptinite macerals bituminite and alginite predominate and are diagnostic macerals of this unit the amount of bituminite locally exceeds 85 vol other liptinite macerals such as sporinite and liptodetrinite are present in significantly lower amounts one exception being asm which may be present in higher amounts humic constituents vitrinite and inertinite are rare present in small amounts in the kupferschiefer beds the organic matter composition points to type ii kerogen for this unit

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

the structure of three dimensional convection beneath oceanic spreading centres summary buoyant flow beneath oceanic spreading centres can be driven by density gradients due to thermal expansion and compositional gradients arising from the extraction of melt numerical experiments are used to examine the pattern of mantle flow and melting produced by these two sources of buoyancy compositional buoyancy leads to three dimensional flow beneath the spreading axis with along axis variations in upwelling and melt production thermal buoyancy drives axis parallel circulation and convective rolls aligned perpendicular to the spreading axis that originate at a distance off axis controlled by the spreading rate and mantle viscosity hie distance from the axis to the onset of observed sea floor gravity lineations may show a similar dependence on spreading rate compositional density stratification suppresses the formation of thermally driven rolls near the axis and when it is large causes the thermal rolls to become unstable leading to time dependent behaviour the amount of crust produced is less sensitive to spreading rate than in passive flow models however it has a maximum at intermediate spreading rates due to the presence of thermally driven upwelling in the melting region along axis variations in upwelling and melt production suggest that buoyant flow may have an important effect on the segmentation of spreading centres particularly at slow spreading rates

spatial and temporal variations of subsidence of the east pacific rise 0 23 s abstract the distributions of crustal depths as a function of age have been analysed for the southeast pacific region along the east pacific rise between the equator and the easter microplate 23 s using age data and a new compilation of bathymetric data subsidence rates for both eastern and western flanks asymmetry of subsidence and zero age depths are computed within flow line corridors on the nazca and pacific plates variations of subsidence rates axial depths and subsidence asymmetry are examined both in space within corridors and time within several age intervals the variability in these parameters along the strike of the east pacific rise is systematic and serves to define several orders of ridge segmentation the largest variations of these parameters are correlated with the large scale segmentation of the ridge axis i e transform faults and very large overlapping spreading centres and are interpreted as related to variations in mantle heterogeneities mainly dependent upon temperature smaller variations of subsidence parameters are correlated with second and sometimes third order segmentation of the ridge axis which could be related to variations in axial magmatic supply across strike variations of subsidence suggest the existence of small lateral temperature and density variations in the mantle when analysing the slope of the distribution of depth versus square root of age within corridors we have observed the existence of changes in the slope which occur at specific age limits we have estimated the subsidence over different age ranges in order to determine the temporal evolution of subsidence parameters rates and asymmetry such an analysis may inform on the past axial segmentation and on the persistence of axial discontinuities in time a linear relationship between subsidence rates and axial depths is determined for each age range and suggests that shallower segments subside faster than deeper segments although a similar statistically defined linear relationship exists for any mid ocean spreading ridge both for intermediate or fast ultrafast spreading the resultant slopes of this relationship vary from ocean to ocean and show that this relationship is not universal over all oceans

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

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the southern east pacific rise between february 1991 and may 1992 33 intraplate earthquakes having body wave magnitudes between 4 3 and 6 0 were located on the west flank of the southern east pacific rise by the international seismological center seven months after the last teleseismic event we deployed four ocean bottom seismometers at the site of the teleseismic swarm one hundred and ninety two microearthquakes were located using p and s travel times of events recorded by three or more instruments during the 16 day deployment most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains in addition several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis the focal depths of the microearthquakes range from 1 to 15 km and most are between 5 and 12 km similar to the depth range of the teleseismic events hung and forsyth 1996 the composite p wave polarities indicate that the microearthquakes had a variety of focal mechanisms we developed a new grid search inversion technique that utilizes the p wave polarities and the empirically corrected ratios of p and s wave amplitudes to find the focal mechanisms of individual events within the acceptable travel time and amplitude misfits focal solutions are fairly stable normal faulting is found in the ridge parallel seismicity line the thrust and strike slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events there is no apparent depth dependence of fault types none of the existing models of the sources of stress ridge push thermoelastic stresses loading by local topographic features caldera collapse and north south extension of the pacific plate provides a satisfactory explanation for both the teleseismic swarm and microearthquakes we propose a new tectonic scenario in this scenario the lithosphere is prestressed by the cooling of the plate magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere which is already under extensional thermal stress producing the larger teleseismically detected events once the dikes propagate into the lithosphere the region surrounding the dikes behind the tips is compressed by the overpressure of magma depending on the geometry of the dikes the local orientations of the minimum principal stress and the local weaknesses in the lithosphere thrust or strike slip faulting the microearthquakes may occur

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

morphotectonic characteristics distribution and probable genesis of bathymetric highs off southwest coast of india abstract the western continental margin of india and the adjacent deep ocean basins were formed by break up and separation among india seychelles and madagascar since the late cretaceous the initial india madagascar separation and the subsequent india seychelles separation are believed to have been caused by the marion hotspot at 90 ma and the reunion hotspot at 68 5 ma respectively these geodynamic events resulted in the formation of several bathymetric highs that probably represent imprints of these volcanic events in the present study these bathymetric high features were mapped comprehensively to understand their morphotectonic characteristics using a fresh set of multibeam bathymetry sea surface gravity and magnetic anomalies complemented by the available multichannel seismic reflection sections a high resolution bathymetric map of the southwestern continental margin of india and the adjoining deep offshore regions has been generated to decipher detailed morphological configuration and distribution of prominent undersea bathymetric features we also carried out detailed morphometric analysis of these features to deduce the morphological parameters a total of 33 individual bathymetric high features were identified and classified as seamounts hills knolls guyots and plateaus based on the standardization of undersea feature names published by intergovernmental oceanographic commission ioc and international hydrographic organization iho in 2013 multichannel seismic reflection sea surface gravity and magnetic data were used to describe the sub seafloor configuration and qualitative interpretation of the geophysical signatures associated with the bathymetric highs interpretation of the multichannel seismic reflection sections suggest that some of these identified features are extrusive in nature while others are intrusive these features are associated with characteristic gravity highs superimposed over regional negative anomalies and complex negative and positive magnetic anomalies the study results suggest that the genesis of the bathymetric highs mapped in the study area could be attributed to the hotspot volcanism caused by the marion or reunion hotspots we infer that the features in the southwestern continental margin of india closer to the alleppey trivandrum terrace complex might have been created by the marion hotspot volcanism while those in the laccadive basin and eastern sector of the laccadive plateau might have been formed by reunion hotspot volcanism

high resolution surveys along the hot spot affected galapagos spreading center 2 influence of magma supply on volcanic morphology the galapagos spreading center gsc at 89 95 w exhibits large gradients in magma supply at a relatively constant intermediate spreading rate making this area an ideal natural laboratory to study the effects of magma supply on volcanism at seafloor spreading ridges prior work shows that the gsc develops from axial valley to shallow axial rise and a shallow magma sill much like a typical fast spreading ridge as the contribution of the hot spot increases the volcanic morphology varies with magma supply in a predictable manner that we divide into three terrains based on the characteristic style of volcanic emplacement and edifice construction within each terrain the volcanic cone terrain comprises most of the gsc and is characterized by prominent volcanic cones within a 1 km wide and 100 m deep axial graben approaching the area of maximum mantle plume influence at 91 w the gsc axis lies along an elevated axial rise split by a 1 km wide and 100 m deep axial graben and the style of volcanism shifts to axial volcanic ridge terrain characterized by axis elongate low relief ridges of pillow lava the lava channel terrain comprises only one segment on either side of the maximum magma supply at 91 w where sheet lava flows and lava channels are relatively widespread a general lengthening of seafloor fissures with increasing magma supply suggests a greater tendency toward linear source eruptions in agreement with the volcanic observations these results suggest that magma supply rather than magma chamber depth or rate of tectonic extension is the primary influence on lava morphology hence eruptive processes at seafloor spreading ridges in general in both the axial volcanic ridge and lava channel terrains a single prominent volcanic cone exists within each volcanic segment suggesting a segment centered magma focusing

photogeologic study of a segment of the east pacific rise axis near 21 n latitude geologic and structural interpretations based primarily on photographic data collected by the marine physical laboratory deep tow instrument the woods hole oceanographic institution angus and the french manned submersible cyana show that extensional tectonic activity modifies and locally replaces extrusive volcanic processes along a 6 km segment of the east pacific rise axial zone near lat 21 n relatively young pillow flows dominate the spreading axis however fissures are present throughout the axial zone and there is a light dusting of pelagic sediment on even the youngest flows in contrast the axial zone immediately southwest in the rise project study area is free of fissures and sediment photographic evidence for the existence of undiscovered active hydrothermal vents or narrow zones of diffuse hydrothermal emissions includes anomalously high concentrations of common deep sea organisms not the exotic hydrothermal vent communities some of these are coincident with high 3 he values and positive temperature anomalies discovered in deep tow water samples lupton and others 1980 estimation of the absolute age of pillow flows based on the sedimentation rate and the volume of interpillow sediment ponds suggests that the average pillow flow with a sediment cover of 70 is approximately 1 000 yr old and that a pillow flow will be completely covered with sediment in about 40 000 yr this relation indicates that the extrusive terrane of the inner axial zone in the study area although undergoing extensional modification is considerably less than 1 000 yr old visual geologic data collected by the several deep sea photographic systems are complementary in nature and the best geologic interpretations are made through use of more than one system to provide various photographic perspectives and scales of observation

are oceanic fracture zones locked and strong or weak new evidence for volcanic activity and weakness traditionally oceanic fracture zones and other lithospheric and crustal faults have been viewed as zones of weakness that are vulnerable to seismic and volcanic activity this notion has recently been questioned on the basis of evidence showing that initial depth offsets scarp height across some pacific fracture zones are preserved even after 100 m y of differential subsidence this evidence suggests that some parts of oceanic fracture zones are locked shortly after they drift away from ridge transform intersections because the fracture zones seem to be strong welds differential cooling stresses cause flexure in the lithosphere on both sides of the fracture in this paper we briefly review the seemingly contradictory evidence for the thermomechanical characteristics of oceanic fracture zones locked and strong vs slipped and weak and present new evidence indicating that parts of some fracture zones are volcanically active volcanism is an indication of relative weakness because strain and failure are required to open volcanic conduits this and other evidence for the inherent weakness of fracture zones can be reconciled with evidence for strength if different segments of fracture zones exhibit contrasting thermomechanical behavior more data are needed to determine whether this is so and whether there are systematic patterns of distribution for weak and strong parts of individual fracture zones

buoyant mantle upwelling and crustal production at oceanic spreading centers on axis segmentation and off axis melting numerical experiments are used to examine the effect of buoyant mantle flow on melt generation near spreading centers buoyancy results from 1 the depletion of residual mantle in fe relative to mg mantle depletion buoyancy 2 the presence of low density melt melt retention buoyancy and 3 thermal expansion thermal buoyancy thermal buoyancy drives off axis convective rolls that develop closer to the spreading axis at slower spreading rates melt retention buoyancy drives focused upwelling into isolated melting centers with increasing spreading rate the amplitude of on axis crustal segmentation decreases the melting region being essentially two dimensional at full spreading rates greater than 100 km m y these numerical experiments predict centers of mantle upwelling that are more widely spaced than the actual wavelength of inferred crustal thickness variations this discrepancy indicates that short wavelength segmentation of slow spreading centers requires some process not included in our models of mantle flow at fast spreading rates when the lithosphere is thinner melt retention buoyancy gives rise to isolated focused centers of off axis upwelling and melt production off axis melting centers begin to form at about 300 km from the axis with a characteristic along axis spacing of 200 km successive off axis melting centers are shifted along axis so that crustal thickness far from the axis is increased at a 100 km wavelength

east pacific rise 18 19 s asymmetric spreading and ridge reorientation by ultrafast migration of axial discontinuities a detailed bathymetric side scan and magnetic survey of the east pacific rise out to a seafloor age of 1 ma has been carried out between 18 and 19 s it reveals that some left stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates 1000 mm a or higher these rapid migration events have left on the nazca plate discordant features striking nearly parallel to the ridge axis a discontinuity with an offset of several kilometers has migrated in two stages at around 0 45 and 0 3 ma and has left two large discordant zones consisting of a series of unfaulted hummocky basins bounded to the east by short ridges oriented about n s oblique to the ambient 013 fabric the morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large slowly migrating overlapping spreading centers between 18 35 and 19 03 s the ridge axis is flanked a few kilometers to the east by a prominent sedimented ridge previously recognized as a recently abandoned ridge axis the present ridge segment steadily deepens and narrows southward which suggests the abandoned ridge has been rafted onto the nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump by transferring pacific lithosphere to the nazca plate these migration events account for most of the asymmetric accretion observed faster to the east this process is consistent with the features common to asymmetric spreading namely the sudden onset or demise of asymmetric spreading and the ridge segment to ridge segment variability because the discordant zones left by these rapid migration events are near parallel to the ambient seafloor fabric they are unlikely to be detected by conventional bathymetry or magnetic surveys and so called ridge jumps may actually often represent ultrafast propagation of a ridge segment variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0 8 m y instead the counterclockwise trend of the east pacific rise relative to the brunhes matuyama reversal 0 78 ma mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the pacific lithosphere to the nazca plate at 18 s than at 19 s in keeping with the regional features of the magnetic anomalies we propose that an 8 to 10 km left stepping discontinuity which was located between 17 and 17 30 s at 0 78 ma has been recently redistributed into the present staircase of small left stepping discontinuities between 16 and 19 s this smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra fast propagation episodes and may be the consequence of a significant replenishment of the magma reservoir between 17 and 17 30 s during the past 1 m y

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

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

monochromatic t waves from underwater volcanoes in the pacific ocean ringing witnesses to geyser processes abstract we analyze swarms of exceptionally intense and sustained t waves recorded during 1991 through 1994 at the polynesian seismic array the strongest swarm lasted 15 months and originated southwest of the eltanin fracture zone in the immediate vicinity of a documented seamount associated with a recognized volcanic chain branching out of the local segment of the mid oceanic ridge the eltanin t waves are characterized by an exceptionally monochromatic spectrum featuring a single line of a few hertz frequency in the range 2 to 80 hz while no similar characteristics had previously been observed in 25 yr of recording in polynesia comparable spectra were recorded during a 1993 swarm at the revilla gigedo islands following by 4 months a documented eruption of socorro island in both cases we interpret the t waves as evidence of a major underwater volcanic process the existence of a single resonating frequency and in particular the absence of any overtones is generally not compatible with the now classical models of the resonance of a fluid filled crack and we speculate that the source of the phenomenon may be the oscillation of a bubbly liquid which could result from vaporization of seawater in the presence of a large lava lake

lessons from venus for understanding mantle plumes on earth abstract mantle plumes are important in the magmatic and tectonic history for both earth and venus the expression of plumes is distinctive on venus and complementary to that on earth therefore a cross comparison is useful for better understanding plume magmatism on both planets in contrast to the earth venus has no observed record of plate tectonics a low degree of surface erosion and an apparently short duration for the formation of the present planetary surface the absence of plate tectonics indicates that all magmatism is intraplate and is generated beneath a stagnant lithospheric lid a low degree of surface erosion preserves the surface structures and short wavelength topography the short duration of preserved magmatic activity suggests a global resurfacing event magmatic elements include a individual volcanoes with diameters ranging up to 1000 km which represent hotspots b annular structures termed coronae with diameters averaging 300 km but ranging up to 2600 km and which appear to lack terrestrial i e earth analogues c radiating graben fissure systems extending up to 2000 km in radius some of which are purely uplift related while others mark the plumbing system dyke swarms of volcanic systems d lava flow fields of scale comparable to terrestrial flood basalts large igneous provinces lips and e regions of small shield volcanoes representing shallow source melting there are several hierarchies of magmatic events on venus ordered in terms of increasing scale and significance 1 isolated coronae volcanoes flow fields and radiating graben systems 2a individual and small clusters of volcanoes and coronae associated with topographic swells geoid highs and triple junction rifting these are most clearly indicative of terrestrial type plumes originating from the deep mantle 2b coronae distributed along rifts chasmata these are the clearest examples of melt generation associated with rifting 3 regional concentration of activity in the beta atla themis bat region this is the closest example of a plume cluster event sometimes termed a superplume event and 4 global volcanic resurfacing of the volcanic plains no terrestrial analogue is confirmed although the global burst of terrestrial plume activity in the neoarchean is a possible analogue

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

constraints on a buoyant model for the formation of the axial topographic high on the east pacific rise bathymetry and gravity data from four geophysical surveys along the 15 19 s superfast spreading section of the east pacific rise are used to study formation of the axial topographic high after removing a best fitting thermal model the averaged residual axial topography is 265 m high and 15 km wide the residual gravity shows no evidence for shallow isostatic compensation of this topography suggesting deep rooted compensation or dynamic support previous models postulate that the axial high is compensated by low densities in a zone of partial melting within the upper mantle we model the physical properties of the lithosphere and asthenosphere required to match the residual gravity and bathymetry using a nonlinear inversion assuming the high is uplifted by buoyancy forces from low density mantle deep rooted buoyancy forces cannot create a narrow axial high unless they occur in a low viscosity conduit surrounded by high viscosity mantle this high viscosity region could be created by depletion and extraction of all water during the melting process we find that the viscosity of the asthenosphere beneath the melt conduit must also be lower than the viscosity of the residual mantle surrounding the conduit our most reasonable model has a viscosity of 1020 pa s in the residual mantle surrounding a melt conduit which extends 50 km below the seafloor and a viscosity of 1016 6 pa s in the asthenosphere below the melt conduit the viscosity of the melt conduit must be about 1015 pa s or about 105 times lower than the surrounding residual mantle with a density contrast equivalent to the retention of a small fraction of melt 5 because higher melt concentrations are probably required to create even small changes in viscosity and the viscosity of the undepleted mantle is probably higher than 1016 6 pa s we conclude that it is unlikely that the axial topographic high is formed by buoyancy forces in the crust and mantle

secondary convection and the growth of the oceanic lithosphere abstract this paper presents a study of the evolution of the oceanic lithosphere from a thermomechanical approach we have investigated the finite amplitude development of secondary convection cells beneath the oceanic plates by means of the single mode mean field equations and the fully 2 dimensional convection equations using finite element techniques both newtonian and non newtonian rheologies with highly temperature and pressure dependences and an activation energy of 100 kcal mol 1 have been employed the temperature at the base of the convecting medium governs the final thickness of the lithosphere the mean interior temperature varies only slightly during the temporal evolution non newtonian rheology has the tendency to induce oscillatory time dependent behavior of the flow heat flow topography and gravity are influenced by secondary convection in two ways small scale perturbations with wavelengths of around 600 km arise from the lateral thermal differences between the uprising and descending convective limbs large scale features are also produced as a consequence of lithospheric growth the calculated quantities of the heat flow topography and gravity associated with small scale convection are typically in the range of o hfu o 102m and o 10 mgal the horizontal mean temperature profiles from the convection model are used to calculate long wavelength geophysical observables as a function of age convective processes are found to reduce the rate of lithospheric thickening predictions from our model can fit well the observed data of heat flow ocean floor topography and geoid offsets along fracture zones the last data base exhibiting the most sensitivity to thermal perturbations below the lithosphere our calculations show that the oceanic lithosphere is able to grow continuously up to o 109 y long past the flattening of the seafloor we report here that the thickness of a thermally equilibrated lithosphere could reach 250 km which lies within the appropriate range of values for the continental lithosphere as inferred from studies in seismology flexure observations and secular polar motions

on the source of cross grain lineations in the central pacific gravity field subtle lineations in the marine gravity field of the central pacific derived from seasat altimeter data were observed by haxby and weissel 1986 they suggested that these cross grain lineations were evidence of small scale convection beneath the pacific plate we have examined these features by comparing multiple collinear gravity and bathymetry profiles in the fourier transform domain our nine gravity profiles were each obtained by stacking averaging three or more individual repeat geosat erm altimeter passes prior to stacking the individual geosat passes were fit to a cubic spline and then differentiated along track to produce along track deflections of the vertical or horizontal gravity corresponding bathymetric profiles were produced by projecting onto geosat ground tracks bathymetric observations from six r v thomas washington legs and three r v conrad legs that virtually coincide with these geosat tracks after fourier transforming the resulting gravity and bathymetry profiles we estimate admittances of gravity to bathymetry these admittances are generally low they also tend to be negative at very short wavelengths 50 km they are consistent with models of flexural isostatic compensation by a very thin lithosphere approximately 2 km they are not consistent with models of dynamic compensation we suggest therefore that either 1 these cross grain lineations began to form very near the east pacific rise or 2 they formed on older anomalously weak lithosphere we also suggest that the gravity lineations result primarily from loads beneath the seafloor in combination with secondarily loads on the seafloor depths of these subseafloor loads appear not to exceed significantly typical moho depths

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

modal depth anomalies from multibeam bathymetry is there a south pacific superswell a region west of the southern east pacific rise sepr between the marquesas and austral fracture zones has previously been found to exhibit anomalous depth age behavior based on gridded bathymetry and single beam soundings since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism the magnitude and regional extent of depth anomalies over the young eastern flank of the so called south pacific superswell are re examined using a mode seeking estimation procedure on data obtained from several recent multibeam surveys the modal technique estimates a representative seafloor depth based on the assumption that bathymetry from non edifice and edifice populated seafloor has a low and a high standard deviation respectively flat seafloor depth values are concentrated in a few bins which correspond to the mode this method estimates a representative seafloor value even on seafloor for which more than 90 of coverage is dominated by ridge and seamount clusters where the mean and median estimates may be shallow by hundreds of meters where volcanism related bias is moderate the mode mean and median estimates are close depth age results indicate that there is only a small anomaly 200 m over is 35 ma pacific plate seafloor with little age dependent shallowing suggesting that the lithosphere east of the main hot spot locations on the superswell is normal an important implication is that in sparsely surveyed areas depths from etopoj are significantly different from true depths even at large scales 1000 km and thus are unsuitable for investigations of anomalies associated with depth age regressions we find that seafloor slopes on conjugate profiles of the pacific and nazca plates from i5 to 35 ma are both slightly lower than normal but are within the global range proximate to the sepr seafloor slopes are very low 218 m myr 2 on the pacific plate o 22 ma and slightly high 410 m myr i21 on the nazca plate o 8 ma slopes for older pacific seafloor 22 37 ma are near normal 399 m myr i seafloor slopes are even lower north of the marquesas fracture zone but are highly influenced by the marquesas swell we find that the low subsidence rate on young pacific seafloor cannot be explained by a local hot spot or a small scale convective model exclusively and a stretching thickening model requires implausible crustal thickness variation 30

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

location of louisville hotspot and origin of hollister ridge geophysical constraints abstract the application of a new geometric technique p wessel l kroenke a geometric technique for relocating hotspots and refining absolute plate motions nature 387 1997 365 369 recently pointed to a recent change in the pacific plate absolute motion and suggested that the louisville hotspot could now be located underneath the hollister ridge south of the eltanin fault system however the pole that was proposed for the last 3 ma does not fit the trend of most pacific volcanic alignments supporting geochemical evidence i vlastelic l dosso h guillou l geli h bougault j etoubleau j l joron geochemistry of the hollister ridge relation with the louisville hotspot and the pacific antarctic ridge earth planet sci let 160 1998 777 793 that does not favor a genetic relationship between the louisville hotspot and the hollister ridge we propose a pole near 57 n 100 w that reconciles kinematic models with a previously proposed location p lonsdale geography and history of the louisville hotspot chain in the southwest pacific j geophys res 93 1988 3078 3104 for the louisville hotspot near a pleistocene volcano dredged at 50 5 s 139 2 w and claim that the hollister ridge most probably results from intraplate deformation processes

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

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

placing bounds on lithospheric deformation in the central pacific ocean abstract lineations in the gravity field in the central pacific ocean have been explained by models of both small scale convection and lithospheric boudinage the boudinage model requires deformation of the plate and we have investigated bounds on the amount of deformation we calculate separations between a pair of fracture zones on the pacific plate which occur in an area where the lineations are well developed and compare these values to the separation of the corresponding fracture zone pair on the nazca plate where no lineations have been observed satellite and surface ship data was used to locate fracture zones and small circles were fitted to sections of picks which were chosen around their points of intersection with magnetic isochrons great circle distances from these small circles to stage poles were calculated and the difference between these distances gave the separations between the fracture zone pairs we found that between anomalies 13 and 7 the difference in fracture zone spacing on the pacific plate compared to the nazca plate appears to increase as the age of oceanic lithosphere decreases the maximum difference calculated was 97 km about 4 of the distance between the fracture zones and we attribute this difference to the re orientation of the ancient pacific farallon spreading centre between anomalies 7 and 5 the main lithospheric stretching models require the fracture zone separation to increase with age but this is not seen in our results

shipboard geophysical indications of asymmetry and melt production beneath the east pacific rise near the melt experiment near the mantle electromagnetic and tomography melt experiment seamounts form and off axis lava flows occur in a zone that extends farther to the west of the east pacific rise than to the east indicating a broad asymmetric region of melt production more seamounts slower subsidence and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west variations in axial ridge shape axial magma chamber continuity off axis volcanism and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15 s than beneath 15 degrees55 s recent volcanism occurs above mantle with the lowest seismic velocities

non hotspot formation of volcanic chains control of tectonic and flexural stresses on magma transport the south pacific in the vicinity of both the superswell and the east pacific rise is repleat with volcanic chains that unlike the emperor hawaiian chain defy the hypothesis of formation via the relative motion of plates and hotspots we propose two nearly identical models for the origin of near axis and superswell chains assuming that both regions are underlain by significant quantities of more or less uniformly distributed partial melt given an initial volcanic load or a local anomaly in the melt source region volcanic chains form by magmatic hydrofracture at local tensile maxima of flexural and membrane stresses fracture wall erosion by magma flow provides a feedback which results in discrete edifices within the chains the model predicts island chains aligned with a deviatorically tensile tectonic stress near the ridge the elastic lithosphere is thin and observations and theoretical considerations indicate a strong deviatorically tensile stress field perpendicular to the ridge axis under such conditions the model results in parallel lines of volcanoes perpendicular to the spreading ridge later interstitial volcanism within the individual chains reduces the average spacing and results in nearly continuous ridges on the thicker lithosphere of the superswell membrane stresses are negligible and the model produces chains of much more widely spaced volcanoes a more isotropic stress field may result in broader chain like patterns of volcanoes in both cases the chains represent self propagating disturbances the resulting age progressions are thus independent of plate velocity but depend only on the dynamics of volcano formation and evolution

record of seamount production and off axis evolution in the western north atlantic ocean 25 25 27 10 n using multibeam bathymetry we identified 86 axial and 1290 off axis seamounts near circular volcanoes with heights 70 m in an area of 75 000 km2 on the western flank of the mid atlantic ridge mar 25 25 n to 27 10 n extending 400 km from the inner rift valley floor to 29 ma crust our study shows that seamounts are a common morphological feature of the north atlantic seafloor seamount producing volcanism occurs primarily on the inner rift valley floor and few if any seamounts are formed on the rift valley walls or the ridge flank the high abundance of off axis seamounts is consistent with 1 3 km wide sections of oceanic crust being transferred intact from the axial valley to the ridge flank on crust 4 ma significant changes in seamount abundances sizes and shapes are attributed to the effects of faulting between 0 6 and 2 m y off axis in the lower rift valley walls few seamounts are completely destroyed by inward facing faults and population abundances are similar to those on axis however faulting reduces the characteristic height of the seamount population significantly in the upper portions of the rift valley on 2 4 ma crust crustal aging processes sedimentation and mass wasting together with additional outward facing faults destroy and degrade a significant number of seamounts beyond the crest of the rift mountains 4 ma crust faulting is no longer active and changes in the off axis seamount population reflect crustal aging processes as well as temporal changes in seamount production that occurred at the ridge axis estimates of population density for off axis seamounts show a positive correlation to crustal thickness inferred from analysis of gravity data suggesting that increased seamount production accompanies increased magma input at the ridge axis we find no systematic variations in seamount population density along isochron within individual ridge segments possible explanations are that along axis production of seamounts is uniform or that seamount production is enhanced in some regions e g segment centers but many seamounts do not meet our counting criteria because they are masked by younger volcanic eruptions and low relief flows

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

characteristics and implications of isoprenoid and hydroxy tetraether lipids in hadal sediments of mariana and yap trenches abstract as one of the least explored regions on earth the hadal zone remains poorly understood in terms of sedimentary organic geochemistry this study examined the composition of isoprenoid glycerol dialkyl glycerol tetraethers iso gdgts and hydroxy gdgts oh gdgts in sediment cores with water depths ranging from 5058 to 10 911 m from the mariana and yap trenches the results showed a higher proportion of gdgt 0 25 73 mean 49 in the trench iso gdgts pool than those from shallower ocean counterparts 5000 m revealed a significant negative linear correlation with water depth tex 86 h r2 0 89 p tex 86 l r2 0 89 p

analysis of gravity and topography in the glimpse study region isostatic compensation and uplift of the sojourn and hotu matua ridge systems 1 the gravity lineations intraplate melting petrologic and seismic expedition glimpse experiment investigated the formation of a series of non hot spot intraplate volcanic ridges in the south pacific and their relationship to cross grain gravity lineaments detected by satellite altimetry using shipboard gravity measurements and a simple model of surface loading of a thin elastic plate we estimate effective elastic thicknesses ranging from 2 km beneath the sojourn ridge to a maximum of 10 km beneath the southern cross seamount these elastic thicknesses are lower than predicted for the 3 9 ma seafloor on which the volcanoes lie perhaps due to reheating and thinning of the plate during emplacement anomalously low apparent densities estimated for the matua and southern cross seamounts of 2050 and 2250 kg m 3 respectively probably are artifacts caused by the assumption of only surface loading ignoring the presence of subsurface loading in the form of underplated crust and or low density mantle using satellite free air gravity and shipboard bathymetry we calculate the age detrended residual mantle bouguer anomaly rmba the rmba corrects the free air anomaly for the direct effects of topography including the thickening of the crust beneath the seamounts and volcanic ridges due to surface loading of the volcanic edifices there are broad negative rmba anomalies along the sojourn and brown ridges and the hotu matua seamount chain that extend nearly to the east pacific rise these negative rmba anomalies connect to negative free air anomalies in the western part of the study area that have been recognized previously as the beginnings of the cross grain gravity lineaments subtracting the topographic effects of surface loading by the ridges and seamounts from the observed topography reveals that the ridges are built on broad bands of anomalously elevated seafloor this swell topography and the negative rmba anomalies contradict the predictions of lithospheric cracking models for the origin of gravity lineaments and associated volcanic ridges favoring models with a dynamic mantle component such as small scale convection or channelized asthenospheric return flow

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

jasper seamount seven million years of volcanism jasper seamount is a young mid sized 690 km sup 3 oceanic intraplate volcano located about 500 km west southwest of san diego california reliable sup 40 ar sup 39 ar age data were obtained for several milligram sized samples of 4 to 10 ma plagioclase by using a defocused laser beam to clean the samples before fusion gee and staudigel suggested that jasper seamount consists of a transitional to tholeiitic shield volcano formed by flank transitional series lavas overlain by flank alkalic series lavas and summit alkalic series lavas twenty nine individual sup 40 ar sup 39 ar laser fusion analyses on nine samples confirm the stratigraphy 10 3 10 0 ma for the flank transitonal series 8 7 7 5 ma for the flank alkalic series and 4 8 4 1 ma for the summit alkalic series the alkalinity of the lavas clearly increases with time and there appear to be 1 to 3 m y hiatuses between each series the age data are consistent with the complex magnetic anomaly of jasper however the dominant reversed polarity inferred from the anomaly suggests that most of the seamount formed at ca 11 ma prior to the onset of chron c5n the duration of volcanism of jasper seamount is slightly longer than the duration of volcanism at hawaiian volcanoes suggesting thatmore individual age data from seamounts may constrain the age of a seamount only to within about 7 m y unless the stage of volcanism can be unambiguously determined extrapolating from the results of our study similar precision in age determinations should be possible on 50 mg of 1 ma plagioclase from mid ocean ridge basalt opening new possibilities in the geochronology of young low potassium volcanic rocks less

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

trade off in production between adjacent seamount chains near the east pacific rise bathymetric data and side scan sonar images collected near the southern east pacific rise between 15 and 19 s1 2 have revealed many seamounts on the west flank of the rise most of which are organized into chains running perpendicular to the rise axis the number of seamounts of 5 km diameter is at least twice that expected for average east pacific sea floor3 the formation of numerous small 5 myr linear seamount chains and the tendency for fresh lava flows to be found primarily at the near ridge ends of the chains suggest that there are plume like sources in the upper mantle which are relatively stationary active for extended periods and affected by adjacent sources

evidence for diffuse extension of the pacific plate from pukapuka ridges and cross grain gravity lineations satellite altimeter measurements of marine gravity reveal 100 to 200 km wavelength lineations over a wide area of the pacific plate oriented roughly in the direction of absolute plate motion at least three mechanisms have been proposed for their origin small scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere diffuse n s extension of the lithosphere resulting in lineated zones of extension boudins and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism recently several chains of linear volcanic ridges have been found to be associated with the gravity lineations following ridgelike gravity signatures apparent in high resolution geosat gravity measurements we surveyed a series of volcanic ridges that extend northwest from the east pacific rise flank for 2600 km onto 40 ma seafloor our survey data as well as radiometric dates on samples we collected from the ridges provide tight constraints on their origin 1 individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion 2 the ridges formed along a band 50 to 70 km wide in the trough of one of the more prominent gravity lineations 3 radiometric dates of the largest ridges show no hotspot age progression moreover the directions predicted for minihotspot traces older than 24 ma do not match observed directions of either the gravity lineations or the ridges based on this last observation we reject the minihotspot model the occurrence of the ridges in the trough of the gravity lineation is incompatible with the small scale convection model which would predict increased volcanism above the convective upwelling we favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere however the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model perhaps the fault scarps are buried under more than 30 m of sediments and or covered by the elongated ridges finally we note that if ridge push force is much smaller than trench pull force then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion

transition from seamount chain to intraplate volcanic ridge at the east pacific rise a number of large submarine intraplate volcanic ridges have been discovered throughout the south pacific basin but their origins are enigmatic recent shipboard geophysical surveys reveal that the sojourn ridge one of these large intraplate ridges becomes a chain of discrete seamount volcanoes that intersects the ridge axis this seamount chain exhibits several features that suggest that it is directly related to the sojourn ridge the sojourn seamount chain grows continuously in both volume and number of seamounts with distance from the spreading axis several loci of recent volcanic activity along the chain are evident in the side scan imagery and a mantle bouguer anomaly low underlies the entire length of the chain this evidence provides new constraints on the origin of intraplate volcanic ridges the continuation of the sojourn ridge system as a volcano chain that extends to within 5 km of the spreading axis implies active generation of magma and a focusing mechanism such as flexural stresses induced by the mass of the volcanic pile as the probable mechanism for developing volcanic ridges and long seamount chains

plume formation in the d double prime layer and the roughness of the core mantle boundary numerical simulations of the formation of thermal plumes in the d layer at the base of the earth s mantle show that plumes are initiated by coalescence of small scale convective instabilities within the low viscosity region immediately above the core mantle boundary cmb these instabilities support topographic roughness on the cmb having horizontal scales of 20 50 km and provide a source for scattered p waves seen as precursors to the phases pkikp and pkkp the calculated structure of fully developed plumes emerging from d consists of 5 50 cm yr 1 flow confined to 50 100 km thick vertical conduits with strongly temperature dependent viscosity plumes exhibit time dependent behaviour including upward propagating solitary conduit waves which may contribute to episodicity in hotspot volcanism

the rise flank trails left by migrating offsets of the equatorial east pacific rise axis sea beam and magnetic surveys of the young 1 ma rise flank around nontransform rise crest offsets at 5 5 n 4 9 n 3 4 n 2 0 n and 2 8 s mapped distinctive trails of obliquely lineated highly magnetic crust that spreads from the characteristically curving overlapping rift zones a deep tow survey around the small 1 5 km 3 9 n offset found only normal rise flank terrain near bottom deep tow observations at the medium size 9 km 5 5 n offset and the large 27 km 2 0 n offset provided more detailed information on the formation of crust in the offset trails and of the structure of tectonically modified crust at interrift rise crest sites patches of interrift crust which forms deep overlap basins at medium size offsets and basins and plateaus of rotated abyssal hills at large offsets are periodically shed from the axial zone together with abandoned portions of overlapped rift zone to become important elements of the rise flank trails the oblique azimuths of the paired fracture zone trails that diverge from the rise crest offsets show that most have migrated along the rise crest at average speeds of 25 125 mm yr net movement of the 27 km 2 8 s offset has been slower with alternating periods of rapid northward and southward migration information on the migration history of the surveyed offsets was supplemented by inferring the migration direction of another eight offsets on the equatorial east pacific rise epr whose trails were less thoroughly mapped most offsets of the southern pacific cocos boundary have migrated south and most on the northern pacific nazca boundary have migrated north the patterns are not readily explained by the inferred distribution of mantle upwellings along the epr axis

the regional tectonic fabric of the east pacific rise from 12 50 n to 15 10 n seamarc ii backscatter data for the east pacific rise are used to create structural maps of ridge parallel fabric on the crest and flanks of the rise from 12 50 n to 15 10 n the structural data are statistically analyzed to describe the spacing density azimuth facing direction and length of faults results of the statistical studies are compared with predictions for the width of the zone of active fault formation models for the generation of abyssal hills plate kinematic predictions models of along strike segmentation and observations of the asymmetric subsidence of oceanic crust on the cocos and pacific plates comparisons of the number of faults with distance from the rise crest and examination of the stratigraphic relationship between seamounts and the ridge parallel tectonic fabric illustrate that the zone where new faults are created is located within a few kilometers of the ridge crest fault density data reveal that fault distributions do not resemble periodic processes examination of fault density and spacing data along strike indicates that there is a high probability that the mean number of faults per crustal block differs significantly from north to south changes in along strike statistics correlate well with the occurrence of three overlapping spreading centers located within the survey area additionally fault spacing s determined from seamarc i and seamarc ii backscatter imagery demonstrate that quantitative analyses of seafloor fabric are dependent upon the instrument used to collect the data analysis of inward and outward facing faults indicates that especially in the northern portion of the survey area half graben models are better predictors of abyssal hill morphology than full graben models within the survey area inward facing faults are more abundant and affect more of the oceanic crust than outward facing faults although the cocos and pacific plates subside at different rates this asymmetry is not reflected in the tectonic component of morphology fault azimuth is observed to vary as a function of crustal age although the overall trend of the change in azimuthal values agrees with the trend predicted by relative poles of opening for the pacific and cocos plates the variability in azimuthal data suggests that other processes contribute to the orientation of lineations formed near the axis of the east pacific rise finally the statistical analyses demonstrate that all fault parameters are better correlated about the ridge axis than along strike of the axis the differences between the northern and southern portions of the survey area are reflected in the seamarc ii bathymetric data which depict a continuous narrow ridge crest in the southern region and an irregularly shaped ridge crest in the northern region that shoals and deepens every 30 to 40 km this along strike variability observed over distances of less than 100 km suggests that large scale plate stresses are not the only processes responsible for generating the tectonic fabric observed on the flanks of the east pacific rise

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

linear ridge groups evidence for tensional cracking in the pacific plate a new class of oceanic bathymetric feature discovered in 1987 consists of en echelon groups of linear ridges there are now at least three such sets of ridges known and their remarkable similarity supports the notion that they result from a widespread tectonic process taking place in the interior of plates in two of these major linear ridge groups the crossgrain and puka puka ridges numerous morphologic features indicate that they originated as tension cracks the form of the individual volcanic structures that make up the ridges correlates with the degree of tension that formed them and the similar orientation of all ridges in a group indicates that the stress that formed them is relatively widespread geographically and temporally the ridges show a characteristic sequence of development beginning with a swath of small volcanoes followed by larger domical volcanoes where the ridge will eventually develop this paper examines proposed mechanisms of formation and concludes that no single source of tension in the lithosphere could plausibly have caused both the crossgrain and puka puka ridges the similarity of the orientations of all the groups of en echelon linear ridges on the pacific plate suggests that the individual ridges form normal to a least compressive stress direction that is geographically variable in the crust parallel ridges constrained to a narrow band then result in an en echelon arrangement though some closely spaced synchronously formed sets of two or three ridge segments may have influenced each other s form

convection beneath young oceanic lithosphere implications for thermal structure and gravity small scale convection under the oceanic lithosphere which begins in the first 5 m y of cooling can produce a gravity signal with the amplitude and wavelength observed for large areas of the central pacific and southern indian oceans using seasat altimeter data the trend of the observed anomalies is parallel to the direction of plate motion as might be expected if they were produced by small scale convection models predict that the wavelengths of gravity anomalies increase more rapidly with age than is observed the persistence of short relatively uniform wavelength anomalies 200 km to crustal ages of 50 ma may indicate that they were produced when the lithosphere was very young and thin and were frozen in as cooling thickened the elastic lithosphere small scale convection which begins under very young lithosphere does not violate other geophysical data such as the rate of seafloor subsidence and variations of geoid height with age after convection has begun the subsidence due to thermal contraction within the lithosphere varies linearly with age in the absence of mantle heat sources although the rate of change of these quantities is affected by convection much of the variation of the geoid height across fracture zones can be fit by a model which includes small scale convection

small off axis volcanoes on the east pacific rise abstract a study of sea beam bathymetry and seamarc ii side scan sonar data allows a quantitative measure of the contribution of off axis volcanism to the creation of abyssal hill topography on the east pacific rise epr 9 15 n 9 50 n in order to assess the role of off axis volcanism the distribution of volcanic edifices within 35 km of the ridge was determined we measure the size and location of 55 edifices defined as local highs greater than or equal to 40 m with aspect ratios less than 2 the distribution of small volcanic edifices is notably different from that of larger volcanic constructions seamounts in that the former do not appear to be organized into discrete chains the volcanic edifices form 5 10 km off axis and are typically 40 70 m high in contrast the seamounts 200 m high measured by scheirer and macdonald 1 first form between 5 and 15 km off axis and then continue to grow in volume and size over a distance 2 4 times larger the smaller edifices make a minute contribution to crustal volume 0 02 0 03 but can cover 7 11 of the mature seafloor seamounts contribute 0 3 1 to the volume of oceanic crust and cover only 6 of the seafloor 1 2 we propose that within 10 km of the ridge seamounts and smaller volcanic edifices compete for a limited magma supply the mechanism of melt delivery to seamounts at these distances is not yet fully mature and some magma may leak in a random fashion onto the seafloor to form volcanic edifices typically less than 70 m high beyond 10 km the mechanism of melt delivery to seamounts is mature and no further formation or growth of small volcanic edifices occurs

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

marine geodynamics depth age revisited more than 20 years have passed since the subsidence of the seafloor away from midocean ridges was explained by the cooling of the lithosphere figure 1 a thermal boundary layer at the earth s surface turcotte and oxburgh 1967 although the general concept has proven quite sound and found much practical application the details of the model have never been completely agreed upon for example if no process limits the growth of the thermal boundary layer the seafloor should continue to deepen as the square root of lithospheric age in contrast the observations indicate that little deepening occurs for lithosphere aged greater than about 100 ma ma million years before present early on this fact was explained by supposing that there was a limit to the depth of cooling of the boundary layer such that the lithosphere cools as a plate of finite thickness rather than as a semi infinite half space mckenzie 1967 according to the plate model the depth of old lithosphere approaches a maximum as the geothermal gradient in the plate reaches thermal equilibrium however this theoretical model merely provided a better fit to the observations without addressing the fundamental question of what process might limit the growth of the earth s uppermost thermal boundary layer

distribution of recent volcanism and the morphology of seamounts and ridges in the glimpse study area implications for the lithospheric cracking hypothesis for the origin of intraplate non hot spot volcanic chains 1 lithospheric cracking by remotely applied stresses or thermoelastic stresses has been suggested to be the mechanism responsible for the formation of intraplate volcanic ridges in the pacific that clearly do not form above fixed hot spots as part of the gravity lineations intraplate melting petrology and seismic expedition glimpse project designed to investigate the origin of these features we have mapped two volcanic chains that are actively forming to the west of the east pacific rise using multibeam echo sounding and side scan sonar side scan sonar reveals the distribution of rough seafloor corresponding to recent unsedimented lava flows in the hotu matua volcanic complex recent flows and volcanic edifices are distributed over a region 450 km long and up to 65 km wide with an apparent irregular age progression from older flows in the west to younger in the east the 550 km long southern cross seamount sojourn ridge brown ridge chain appears to have been recently active only at its eastern end near the east pacific rise a third region of recent flows is found 120 km north of southern cross seamount in seafloor approximately 9 myr old no indication of lithospheric extension in the form of faulting or graben formation paralleling the trend of the volcanic chains is found in the vicinity of recent flows or anywhere else in the study area thermoelastic cracking could be a factor in the formation of a few small very narrow volcanic ridges but most of the volcanic activity is broadly distributed in wide swaths with no indication of formation along narrow cracks the sojourn and brown chains appear to begin as distributed zones of small seamounts that later develop into segmented ridges perhaps under the influence of membrane stresses from self loading we suggest that the linear volcanic chains are created by moving melting anomalies in the asthenosphere and that lithospheric cracking plays at most a secondary role

when does small scale convection begin beneath oceanic lithosphere convection on a scale smaller than the horizontal dimensions of lithospheric plates can be produced by instabilities in the upper or lower boundary layer of the larger mantle flow small scale convection associated with the upper boundary layer should pro duce a detectable gravity signal and affect the rate of cooling and subsidence of the lithosphere i describe here a numerical model of small scale convection in a fluid of variable viscosity the results indicate that recently observed gravity anomalies1 showing a pat tern of highs and lows aligned in the direction of oceanic plate motion may be the result of small scale mantle flow the convective flow must begin in the first 6 myr of lithospheric cooling to produce the observed signals which is not inconsistent with constraints on the viscosity of the mantle the calculated trend for the subsidence of the ocean floor is found to be almost linear with time 1 2 even when small scale convection has significantly changed the rate of subsidence for average shallow asthenospheric viscosities of 1018 pa s the model subsidence can match data for the oceans2 3 and reproduce the magnitude and wavelength of the observed gravity anomalies

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

thickening of young pacific lithosphere from high resolution rayleigh wave tomography a test of the conductive cooling model direct seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges on the east pacific rise at 17 s there were two long term broadband ocean bottom seismometer deployments the melt and glimpse experiments these arrays spanned young pacific plate seafloor ranging in age from 0 to 8 ma combining these two data sets we describe the increase in rayleigh wave phase velocities for 16 100 s period as function of distance from the ridge with two parameterizations an arbitrary function of seafloor age and a simple polynomial function of age on the pacific and nazca plates although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the pacific plate can be resolved at periods of 50 s the three parameter polynomial model fits the data almost as well indicating a simple pattern of the evolution of structure with age to compare our observations to the predictions for the conductive cooling of the lithosphere we convert a thermal half space cooling model to shear velocity using anharmonic and anelastic contributions the patterns in the velocities are not consistent with simple conductive cooling the conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range we observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend the asthenospheric low velocity zone is asymmetric dipping to the west with the lowest velocities beneath the pacific plate west of the spreading center the conductive cooling model also over predicts the shear velocities in the lithosphere by 0 2 km s these anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required

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

a three dimensional gravity analysis of the east pacific rise from 18 to 21 30 s multibeam bathymetry and gravity coverage of the east pacific rise epr between 18 o and 21 o 30 s is used to investigate the relation between melt supply and tectonic segmentation at ultrafast spreading rates the long wavelength features in the residual anomaly show a good correlation with those in the bathymetry the highest residual anomaly values occur over the broad discordant zone of the 20 o 40 s overlapping spreading center osc for seafloor ages of 0 ma to at least 1 5 ma we interpret the deepening of the bathymetry and the increase of the residual anomaly toward that discordant zone as due to a decrease of 500 200 m of the crustal thickness hence the 20 o 40 s osc has been associated with a reduced magmatic budget for at least the past 1 5 m y and represents a persistent segmentation of the epr this is consistent with models in which mantle upwelling even at the fastest spreading centers is enhanced between large discontinuities rather than evenly distributed along axis however this decrease of the crustal thickness toward an axial discontinuity is several times smaller than that typically documented for slow spreading ridges which suggests that mantle upwelling is less focused at fast spreading ridges or that along axis transport of crustal material is more efficient or both across the study area the residual anomaly decreases toward the nw by 15 20 mgal this regional gradient can be modeled with lateral temperature variations in the upper mantle of up to 60 o c increasing toward the nw this interpretation is consistent with the numerous seamounts present to the nw and the robust magmatic budget of the ridge between 17 o and 18 o s and it could also explain why the ridge segments defined by the smaller oscs between 18 o and 19 o s propagate very rapidly away from the robust area similar patterns of ridge propagation away from the shallowest section of a ridge have been documented near the galapagos and easter island hot spots hence these shorter ridge segments may not be associated with significant individual melt sources rather they may represent a superficial segmentation due to the interaction between the epr and a mantle heterogeneity located between 17 o and 18 o s

density and size distribution of seamounts in the eastern pacific inferred from wide beam sounding data theoretical expressions relating the distribution of apparent seamount heights per unit length of ship track to the distribution of actual seamount heights per unit area of seafloor are derived for both narrow beam and wide beam echo sounders the shapes of seamounts are approximated as truncated right circular cones of constant slope e height to radius ratio and flatness f i e a seamount of basal radius r has a height h r a flat top of radius fr and a slope angle of arctan e the set of all seamounts in a given physiographic province is assumed to be a poissonian ensemble specified by a function v r the average number of seamounts per unit area for which r r a simple choice for the size distribution of seamounts and one shown to be consistent with data in the eastern pacific is to specify that it decays exponentially with r v r vo exp r in the case of a profile of length l taken in water of depth d with a wide beam echo sounder half beam width the shallow slope deep water approximation e 1 e d yields that the expected number of seamounts with apparent heights h h is n h l 2 1 ed 2 fh lv0e h we apply this sampling theory to apparent height data measured from 44 020 km of wide beam profiles on pacific crust of post mesozoic age although the spatial distribution of seamounts in the eastern pacific is certainly not poissonian the abundance statistics do not involve spatial correlations and we show they are robust with respect to deviations from the poisson model provided the statistics are interpreted as averages over the area actually sampled by the track lines the observed falloff of apparent heights in the eastern pacific is very nearly exponential justifying our use of an exponential model of seamount size distribution the shape parameters are estimated from multibeam surveys of pacific seamounts an analysis of 30 features in the height range 200 4000 m yields 0 21 0 011 and f 0 21 0 028 using these parameters we obtain vo 4 0 1 1 10 9 m 2 and 6 3 0 8 10 4 m 1 these values imply that per 106 km2 in the eastern pacific there are 1600 400 seamounts with summit heights greater than 300 m and 200 60 greater than 1000 m integrated over the entire size distribution seamounts occupy about 6 of the seafloor area and constitute about 0 4 of the oceanic crustal volume our estimates of seamount abundance are appreciably greater than those of most other authors a discrepancy we attribute primarily to the sampling bias of previous studies which generally used uncorrected map counts this hypothesis is corroborated by a comparison of our statistics with map count data for seamounts of heights exceeding 2000 m compiled for the eastern central pacific from the scripps charts above 3000 m the map counts are consistent with the predictions of the exponential model but below this height they are biased to low values

a 220 km2 recently erupted lava field on the east pacific rise near lat 8 s seamarc ii records show a 220 km2 area of unusually high reflectivity on the east pacific rise epr near lat 8 s which we interpret to be a recently erupted lava field this is the only segment of the epr exhibiting high reflectivity over such a large area in our continuous seamarc ii coverage from lat 8 n to 18 n and 3 s to 23 s the lavas appear to issue from the axial neovolcanic zone between lat 8 15 and 8 17 s the lava field extends up to 18km off axis and inundates scarps up to 100 m high on the basis of measurements of scarp heights just outside areas where they are buried we estimate the average thickness of the field to be 70 20 m thus the volume of the lava field is 15 4 km3 in comparison the average annual volcanic budget for the earth is approximately 4 5 km3 and the largest historic basaltic eruption from the icelandic laki fissure in 1783 was approximately 12 3 km3 when did the eruption occur comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the epr suggest that the lava field has little or no sediment cover and is very young this is also one of the only areas on the epr where earthquakes occur that do not appear to be caused by transform faulting if the earthquakes are related to the lava field then major eruptions may have occurred in 1964 1965 and 1969 a large helium plume near lat 13 s may be caused in part by eruptions of the lat 8 s lava field this is consistent with estimates of currents at 2 3 km depth which are such that a plume generated at lat 8 s would eventually appear off axis at lat 13 s our observations suggest that this spreading segment which is bounded by small ridge axis discontinuities at lat 8 05 s and 8 47 s may have been the most volcanically active segment along the epr during the past 25 yr

architecture of on and off axis magma bodies at epr 9 37 40 n and implications for oceanic crustal accretion abstract crustal accretion at fast spreading mid ocean ridges is believed to be concentrated in a narrow zone up to a few kilometers wide centered beneath the ridge axis however there is increasing evidence for off axis magmatism occurring beyond this narrow zone here we present 3d multichannel seismic mcs images from the east pacific rise 9 37 40 n extending to 11 km on the ridge flanks in the axial region two offset axial magma bodies underlie a small ridge axis discontinuity at 9 37 n displaying an overlapping geometry similar to that of the seafloor structures above on the ridge flanks a series of off axis magma lenses oaml are imaged they are located 2 10 km from the ridge axis at 700 to 1520 ms two way travel time below seafloor bsf 1 6 to 4 5 km bsf with variable areas ranging from 0 5 km2 to 5 2 km2 the largest body is centered 4 km east of the ridge axis and is composed of a large continuous flat topped lens and a series of small discontinuous westward dipping bodies along its western edge the flat crest of the oaml lies at approximately the same depth beneath layer 2a as the axial magma lens and we infer that this oaml has formed by aggregation of ascending melts that accumulate at the base of the sheeted dike section a cluster of reflections underlying the oaml at 1260 1510 ms bsf are observed that may be deeper lenses feeding melts to the upper lens this largest oaml is associated with moho travel time anomalies of 120 260 ms within a zone that extends up to 2 km from the edge of the oaml suggesting a lower crust that is partially molten with lower crustal velocities reduced by 8 18 and or thicker than normal by up to 1 km local volcanic edifices are found above two of the three oamls imaged in our study area and are inferred to be the eruptive products of the oamls from the volume of these edifices and the moho travel time anomalies we estimate the potential contribution of off axis magmatism to the total volume of the crust to be 0 01 3 the oamls imaged in our study area are present over roughly the same distance range as the zone of formation of near axis seamounts we speculate that oamls and the volcanic edifices found above them are small scale manifestations of the off axis magmatism that gives rise to near axis seamounts

seamount abundances and abyssal hill morphology on the eastern flank of the east pacific rise at 14 s bathymetric data from a hydrosweep multibeam sonar survey of a 720 km long tectonic corridor on the east flank of the southern epr at 14 14 s covered about 25 000 km of zero age to 8 5 m y old crust magnetic anomaly 4a in this corridor we document a strong correlation of robust along flowline changes in abyssal hill morphology and seamount size distribution with spreading rate changes deduced from our magnetic data indeed we find that both rms height of abyssal hills and abundance and height of seamounts increase significantly as spreading rate changes from 75 mm yr to over 85 mm yr half rate moreover we identified 46 seamounts taller than 100 m previous studies on the southern epr reported a larger density of seamounts organized primarily in chains our investigation however revealed seamounts not associated with major chains leading us to the conclusion that different forms of off axis volcanism occur along the spreading center

morphology and distribution of seamounts surrounding easter island we investigate the morphology and distribution of a seamount population on a section of seafloor influenced by both superfast seafloor spreading and hotspot volcanism the population under investigation is part of a broad chain of seamounts extending eastward from the east pacific rise near easter island in order to define the morphological variability of the seamounts basal shape cross sectional area volume flatness and flank slope are plotted against height for 383 seamounts with heights greater than 200 m based on bathymetry data collected by glori b and seabeam 2000 during three cruises onboard the r v melville in the spring of 1993 nearly complete swath mapping coverage of the seamounts is available for the analysis of size and shape distribution we quantitatively describe the seamount population of this active region in which seamounts cover 27 of the seafloor and account for 4 2 of the total crustal volume over 50 of the total volume 61 000 km3 of seamounts used in this study is made up by the 14 largest seamounts and the remaining volume is made up by the 369 smaller seamounts 200 m in height our analysis indicates there are at least two seamount populations in the easter island salas y gomez island 25 29 s 113 104 w study area one population of seamounts is composed of short seamounts 1200 m shield like pointy cones flatness 1200 m originate exclusively from a hotspot source but only a portion of the smaller volcanoes 1200 m are formed from a hotspot source the remainder would be presumably formed by a normal mantle or mixed source

evidence from en echelon cross grain ridges for tensional cracks in the pacific plate sea floor topography in the pacific is mainly aligned with original spreading directions1 but is overprinted by alignments created by mid plate processes spreading produces abyssal hills and fracture zones and mid plate volcanism generates seamounts isolated or in chains a different category of topography the cross grain discovered in geoid height data collected by the seasat radar altimeter2 comprises linear troughs and swells spaced 200 km apart oblique to fracture zones and abyssal hills but parallel to the hawaiian chain three models have been proposed for the cross grain small scale convection organized into longitudinal rolls by the shear of the pacific plate2 compressive buckling3 and lithospheric boudinage resulting from plate wide tensile stresses4 5 none of the previously available data ruled out any of these models here we report multi beam bathymetric data revealing long narrow en echelon ridges along the cross grain interpreted as evidence of tension cracks in the pacific plate

geosat derived geoid anomalies at medium wavelength geosat profiles of the exact repeat mission have been averaged over a 1 year period and high pass filtered using inverse method techniques the geoid surface constructed with both ascending and descending profiles shows at medium wavelengths band shaped anomalies preferentially elongated in the east west direction these anomalies have an average amplitude of 30 cm and dominant wavelengths to 750km and 1100 km we have performed numerous tests to show that the lineations are not artefacts created by the filtering process moreover two dimensional 2 d filtering with the inverse method applied on a regional basis over the pacific gives essentially similar results indicating that the filtered geoid is not affected by directional bias seafloor topography in the pacific filtered by 2 d inverse method also shows east west trending depth anomalies positively correlated to medium wavelength geoid lineations along the east pacific rise there is a clear correlation between geoid lineations and regional variations in axial depth cross spectral analysis carried out on geoid and topography data over the pacific area gives coherence maxima at 750 km and 1100 km wavelengths and admittance values of 2 3 m km observed admittance matches the magnitude and shape of admittance predicted by lithospheric cooling models suggesting that the lineations are related to regional variations in the plate cooling process convection models produce much higher admittances than observed unless a low viscosity layer is assumed so that dynamical support cannot be completely discarded in most instances however the position and direction of the lineations seem to be controlled by major fractures zones which is in favor of a lithospheric origin in the south central pacific where the lineations appear parallel to absolute plate motion there may be a combination of both lithospheric and sublithospheric processes

geophysical study of the east pacific rise 15 n 17 n an unusually robust segment bathymetric side scan sonar magnetic and gravity data from the east pacific rise epr between 15 and 17 n are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment the axial ridge between the orozco transform fault 15 30 n and the 16 20 n overlapping spreading center osc has an average elevation of 2300 m which is 300 m shallower than typical epr depths and its cross sectional area is double the average value for the northern epr the total opening rate is 86 km myr but the inflated segment appears to have spread faster to the east by more than 20 since 0 78 ma the orientation of magnetic isochrons and lineaments in the side scan sonar indicates a 3 counterclockwise rotation of the spreading direction since 1 8 ma c2 and reflects a change in the pacific cocos plate motion the side scan lineaments also show that the percentage of inward facing faults 83 and the spacing between faults 1 5 km are consistent with the spreading rate dependence shown by carbotte and macdonald 1994 however the mean fault length 4 8 km is 1 5 km shorter than expected for the spreading rate and suggests that extensive off axis volcanism has draped the faults gravity analysis shows that the inflated segment has a 12 mgal bull s eye shaped low in residual mantle bouguer anomaly we offer several possible end member models for the anomaly including a prism of 10 partial melt in the mantle and lower crust or a crustal thickness anomaly of 2 25 km kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0 8 ma and 0 3 ma and have reshaped the plate boundary geometry and inflated the segment

the size distribution of pacific seamounts an analysis of wide beam sea beam and map count data in the eastern and southern pacific confirms the hypothesis that the average number of ordinary seamounts with summit heights h h can be approximated by the exponential frequency size distribution v h vo e h the exponential model characterized by the single scale parameter 1 is found to be superior to a power law self similar model the exponential model provides a good first order description of the summit height distribution over a very broad spectrum of seamount sizes from small cones h 3500 m the distribution parameters obtained from 157 000 km of wide beam profiles in the eastern and southern pacific ocean are vo 5 4 0 65 10 9m 2 and 3 5 0 21 10 3 m 1 yielding an average of 5400 650 seamounts per million square kilometers of which 170 17 are greater than one kilometer in height the exponential distribution provides a reference for investigating the populations of not so ordinary seamounts such as those on hotspot swells and near fracture zones and seamounts in other ocean basins if we assume that volcano height is determined by a hydraulic head proportional to the source depth of the magma column then our observations imply an approximately exponential distribution of source depths for reasonable values of magma and crustal densities a volcano with the characteristic height 1 285 m has an apparent source depth on the order of the crustal thickness