global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

morphology and origin of the osbourn trough the osbourn trough is a 900 km long east west trending gravity low visible in satellite altimetry data which intersects the tonga trench at 25 30 s a recent survey collected gravity magnetic echo sounder and swath bathymetry data on three adjacent north south trending ship tracks centered on the trough the linear gravity low is 20 30 mgal less than the regional value and is accompanied by a flat lying 200 500 m deep sediment filled valley swath bathymetry images reveal several parallel east west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers including a prominent inside corner high and possible pseudo fault trace the presence of magnetic anomalies possibly anomalies 33 and 32 suggests that the seafloor here was formed after the end of the cretaceous normal superchron anomaly 34 these data support the conclusion that this trough is a spreading center which stopped spreading in late cretaceous time the existence of this feature has important implications for tectonic reconstructions in this region the osbourn trough could be part of the fossil spreading center between the pacific plate and a fragment of the phoenix plate the bellingshausen plate this would require the seafloor between the osbourn trough and the chatham rise to the south to be a remnant fragment of the bellingshausen plate this remnant may have joined to the pacific plate when the hikurangi plateau entered the gondwana subduction zone at the chatham rise possibly causing the cessation of spreading on the osbourn trough

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

three dimensional estimation of elastic thickness under the louisville ridge a three dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry this technique is applied to the louisville ridge system to study the tectonic history of the region the inversion is performed as both a first order approximation and a nonlinear relationship between gravity and topography based on parker s 1973 equation while the higher order effect on the gravity anomaly is nearly zero for most of the region the magnitude is significant over the summits of the ridge nevertheless the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region lowering the value by 1 2 km compared with the linear result the incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly resulting in false sidelobe structure of nearly 20 mgal for large seamounts our elastic thickness estimates are compared with the contradictory values obtained in previous studies by cazenave and dominh 1984 and watts et al 1988 we find an increasing elastic thickness along the chain from southeast to northwest with a discontinuity along the wishbone scarp the jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge

ocean island densities and models of lithospheric flexure estimates of the effective elastic thickness te of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones the anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere to erosion of the mechanical boundary layer by mantle plumes to pre existing thermal stresses and to overprinting of old volcanic loads by younger ones a fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads we suggest that load densities have been systematically overestimated in studies of lithospheric flexure potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism we illustrate the effect of underestimating load density with synthetic examples and an example from the marquesas islands this effect combined with the other effects listed above in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses

a seismic study of lithospheric flexure in the vicinity of tenerife canary islands seismic data have been used to determine the crustal and upper mantle structure of tenerife canary islands a volcanic island of tertiary age located on 140 ma oceanic crust reflection data show that oceanic basement dips gently towards the island forming a flexural moat which is infilled by 2 3 km of well stratified material the moat is characterised by a major angular unconformity which we attribute to volcanic loading of pre existing oceanic crust and overlying sediments and the subsequent infilling of the flexure by material that was derived at least in part from the islands refraction data show that the flexed oceanic crust has a mean thickness of 6 41 0 42 km and upper and lower crustal velocities of 4 8 5 4 km s 1 and 6 7 7 3 km s 1 respectively the flexure which has been verified by gravity modelling can be explained by a model in which tenerife and adjacent islands have loaded a lithosphere with a long term 106 yr elastic thickness of approximately 20 km seismic and gravity data suggest that up to 1 5 105 km3 of magmatic material has been added to the surface of the flexed oceanic crust which assuming an age of 6 16 ma for the shield building stage on tenerife implies a magma generation rate of about 0 006 to 0 02 km3 a 1 this rate is similar to estimates from other african oceanic islands e g reunion and cape verdes but is significantly less than that which has been calculated at hawaii there is no evidence in either the seismic or gravity data that any significant amount of magmatic material has underplated the flexed oceanic crust the crustal and upper mantle structure at tenerife therefore differs from other oceanic islands such as hawaii and marquesas where 4 km of underplated material have been reported

no thinning of the lithosphere beneath northern part of the cook austral volcanic chains 1 the cook austral volcanic chains are situated in the south pacific they are composed of several volcanic chains and isolated volcanoes corresponding to distinct events from 55 ma to present time the tectonic history of the volcanoes emplacement is quite complex it involves the activity of at least three hot spots and sometimes several volcanic stages coexist on the same volcano the purpose of this study is to bring new information on the loading history of the northern part of the austral chain by studying the variation of the elastic thickness of the lithosphere we have used the high quality data acquired during the zepolyf2 cruise which conducted in july 1999 a multibeam survey of this area and we got several new ages on seamounts in this region where several volcanic stages overlap the simple elastic model applied on the present day bathymetry is not appropriate we then reconstructed the loading history of each volcano or group of volcanoes using information provided by the radiometric ages and chemical signature of the volcanic stages or the volcanoes morphology we show that the lithosphere is characterized by normal elastic thickness if one correctly isolates the different volcanic stages occurring on this area the abnormally low values of the effective elastic thickness obtained previously over this area may be explained either by not taking into account the overlapping volcanism or by poor quality of the geophysical data used for the determination

new 40ar 39ar age progression for the louisville hot spot trail and implications for inter hot spot motion 1 in this study we present 42 new 40ar 39ar incremental heating age determinations that contribute to an updated age progression for the louisville seamount trail louisville is the south pacific counterpart to the hawaiian emperor seamount trail both trails representing intraplate volcanism over the same time interval 80 ma to present and being examples of primary hot spot lineaments our data provide evidence for an age progressive trend from 71 to 21 ma assuming fixed hot spots this makes possible a direct comparison to the hawaiian emperor age progression and the most recent absolute plate motion apm model wk08g of wessel and kroenke 2008 we observe that for the louisville seamount trail the measured ages are systematically older relative to both the wk08g model predictions and hawaiian seamount ages with offsets ranging up to 6 myr taking into account the uncertainty about the duration of eruption and magmatic succession at individual louisville volcanoes these age offsets should be considered minimum estimates as our sampling probably tended to recover the youngest lava flows these large deviations point to either a contribution of inter hot spot motion between the louisville and hawaiian hot spots or to a more easterly location of the louisville hot spot than the one inferred in the wk08g model both scenarios are investigated in this paper whereby the more eastern hot spot location 52 0 s 134 5 w versus 52 4 s 137 2 w reduces the average age offset but still results in a relatively large maximum offset of 3 7 myr when comparing the new ages to the apm models s04p s04g by steinberger et al 2004 that attempt to compensate for the motion of hot spots in the pacific hawaii or globally hawaii louisville reunion and walvis the measured and predicted ages are more in agreement showing only a maximum offset of 2 3 myr with respect to the s04g model at face value these more advanced apm models which consider both plate and hot spot motions therefore provide a better fit to the new louisville age data the fit is particularly good for seamounts younger than 50 ma a period for which there is little predicted motion for the louisville hot spot and little inter hot spot motion with hawaii however discrepancies in the louisville age distance record prior to 50 ma indicate there is an extra source of inter hot spot motion between louisville and the other pacific hot spots that was not corrected for in the global s04g model finally based on six new 40ar 39ar age dates the 169 w bend in the louisville seamount trail seems to have formed at least 3 myr before the formation of the hawaiian emperor bend the timing of the most acute parts of both bends thus appears to be asynchronous which would require other processes e g plume motions than a global plate motion change between 50 and 47 ma to explain these two observations

gravity inversion using seminorm minimization density modeling of jasper seamount a gravity inversion algorithm for modeling discrete bodies with nonuniform density distributions is presented the algorithm selects the maximally uniform model from the family of models which fit the data ensuring a conservative and unprejudiced estimate of the density variation within the body the only inputs required by the inversion are the gravity anomaly field and the body shape tests using gravity anomalies generated from synthetic bodies confirm that seminorm minimizing inversions successfully represent mass distribution trends but do not reconstruct sharp discontinuities we apply the algorithm to model the density structure of seamounts inversion of the seasurface gravity field observed over jasper seamount suggests the edifice has a low average density of 2 38g cm3 and contains a dense body within its western flank these results are consistent with seismic magnetic and petrologic studies of jasper seamount

internal structure of tenerife canary islands based on gravity aeromagnetic and volcanological data abstract gravity and magnetic methods have been applied to the tenerife island to provide new information about its internal structure for this study 365 gravity stations covering the central part of the island have been selected the anomalous density maps at different depths were obtained by means of an inversion global adjustment on fixed density contrast to describe the three dimensional 3d geometry of the anomalous bodies on the other hand several analysis techniques such as reduction to the pole spectral analysis low pass filtering terrain correction and forward modelling were applied to process the high resolution data obtained in an aeromagnetic survey completed with marine and terrestrial data the joint analysis of gravity and magnetic anomalies has shown tectonic and volcanic features that define some fundamental aspects of the structural framework and volcanic evolution of the island a strong gravity anomaly produced by a large and deep source has been associated with an uplifted block of the basement beneath the southern part of tenerife the sources of the observed gravity highs from 8 km b s l may be associated with the growth of the submarine shield stage that was clearly controlled by regional tectonic the long wavelength magnetic anomalies reveal highly magnetic sources interpreted as gabbro ultramafic cumulates associated with the root zone of a large dyke swarm this intrusive body could be topped by the emplacement zone of magma chambers that correlate with a magnetic horizon at 5 7 0 8 km depth rooted in this highly magnetic zone two dike like structures can be associated with the magmatic feeding system of large recent basaltic volcanoes a shallow magnetic horizon 1 4 km a s l can be correlated with the bottom phonolites of the las canadas edifice in the central part of the island the coincidence of some gravity and magnetic lows is consistent with the presence of low density and low magnetic materials that infill a collapsed caldera system the structures close to the surface are characterised by low density areas connected with the recent volcanism in particular the minimum over the teide volcano hydrothermal alteration is assumed to be the cause of a short wavelength magnetic low over the teide volcano

volcanic evolution of the island of tenerife canary islands in the light of new k ar data new age determinations from tenerife together with those previously published 93 in all provide a fairly comprehensive picture of the volcanic evolution of the island the oldest volcanic series with ages starting in the late miocene are formed mainly by basalts with some trachytes and phonolites which appear in anaga teno and roque del conde massifs in anaga ne three volcanic cycles occurred one older than 6 5 ma a second one between 6 5 and 4 5 ma with a possible gap between 5 4 and 4 8 ma and a late cycle around 3 6 ma in teno nw after some undated units the activity took place between 6 7 and 4 5 ma with two main series separated by a possible pause between 6 2 and 5 6 ma in the zone of roque del conde s the ages are scattered between 11 6 and 3 5 ma between 3 3 and 1 9 ma the whole island underwent a period of volcanic quiescence and erosion the large canadas volcano made up of basalts trachytes and phonolites was built essentially between 1 9 and 0 2 ma to the ne of this central volcano linking it with anaga is a chain of basaltic emission centers with a peak of activity around 0 8 ma the canadas caldera had several collapse phases associated with large ignimbrite emissions there were at least an older phase more than 1 ma old on the western part of the volcano and a younger one less than 0 6 ma old in the eastern side the two large valleys of guimar and la orotava were formed by large landslides less than 0 8 ma ago and probably before 0 6 ma ago the present canadas caldera was formed by another landslide less than 0 2 ma ago this caldera was later filled by the huge teide volcano which has been active even in historic times during the same period a series of small volcanoes erupted at scattered locations throughout the island the average eruptive rate in tenerife was 0 3 km3 ka with relatively small variations for the different eruptive periods this island and la gomera represent a model of growth by discontinuous pulses of volcanic activity separated by gaps often coinciding with episodes of destruction of the edifices and sometimes extended for several million years the neighbouring gran canaria on the other hand had an initial rapid shield building phase during which more than 90 of the island was built and a series of smaller pulses at a much later period a comparison between these three central islands indicates that the previously postulated westward displacement in time of a gap in the volcanic activity is valid only as a first approximation several gaps are present on each island overlapping in time and not clearly supporting either of the models proposed to explain the evolution of the canaries

kuliginite a new hydroxychloride mineral from the udachnaya kimberlite pipe yakutia implications for low temperature hydrothermal alteration of the kimberlites

three dimensional estimate of the lithospheric effective elastic thickness of the line ridge abstract using a new bathymetry grid formed with vertical gravity gradient anomalies and ship soundings bat vgg a 1 1 lithospheric effective elastic thickness te grid of the line ridge was calculated with the moving window admittance technique as a comparison both the gebco 08 and sio v15 1 bathymetry datasets were used to calculate te as well the results show that bat vgg is suitable for the calculation of lithospheric effective elastic thickness the lithospheric effective elastic thickness of the line ridge is shown to be low in the range of 5 5 13 km with an average of 8 km and a standard deviation of 1 3 km using the plate cooling model as a reference most of the effective elastic thicknesses are controlled by the 150 300 c isotherm seamounts are primarily present in two zones with lithospheric ages of 20 35 ma and 40 60 ma at the time of loading unlike the hawaiian emperor chain the lithospheric effective elastic thickness of the line ridge does not change monotonously the tectonic setting of the line ridge is discussed in detail based on our te results and the seamount ages collected from the literature the results show that thermal and fracture activities must have played an important role in the origin and evolution of the ridge

lithospheric deformation induced by loading of the hawaiian islands and its implications for mantle rheology the long term rheological properties of the lithosphere are fundamental for understanding both surface tectonics and mantle dynamics on earth in this study we have developed 3 d finite element models for computing the load induced surface deformation and stress for lithosphere and mantle with realistic nonlinear viscoelastic rheology including the frictional sliding low temperature plasticity and high temperature creep we have determined the lithospheric deformation and stress due to volcano loading in the hawaiian islands region for the last few million years by comparing model predictions with seismic observations of the depth to the top of oceanic crust and depth dependence of seismicity in the hawaiian islands region we have sought to constrain lithospheric rheology our calculations show that the load induced surface deformation is controlled by low temperature plasticity and frictional sliding but is insensitive to high temperature creep lithospheric strength predicted from laboratory derived low temperature plasticity needs to be reduced significantly and a frictional coefficient f ranging from 0 1 to 0 7 is required in order to account for the observations however f 0 1 weakens the shallow part of the lithosphere so much that it causes the minima in strain rate and stress to occur at too large depths to be consistent with the observed depth distribution of seismicity our results therefore suggest a value for f between 0 25 and 0 7 finally the maximum stress that accumulates in the deformed lithosphere beneath the hawaiian islands is about 100 200 mpa for models that match the observations and this stress may be viewed as the largest lithospheric stress on earth

history of the cretaceous osbourn spreading center the osbourn trough is a fossil spreading center that rifted apart the manihiki and hikurangi plateaus during cretaceous time previous models of the osbourn spreading center are based on data collected near the trough axis and therefore only constrain the history of the osbourn spreading center during the last few ma of spreading our data set includes multibeam data collected northward to the manihiki plateau allowing us to examine seafloor morphology created during the entire active period of the osbourn spreading center as well as several additional multibeam data sets that provide the opportunity to examine the relationship between the osbourn paleospreading center and the cretaceous pacific phoenix ridge the axial gravity of the trough is similar to the gravity found at other extinct slow intermediate spreading rate ridges magnetic field measurements indicate that spreading at the trough ceased during chron c34 abyssal hill trends indicate that spreading during the early history of the osbourn spreading center occurred at 15 20 the east west component of this spreading explains the modern east west offset of the manihiki and hikurangi plateaus spreading rotated to 2 5 prior to extinction abyssal hill rms amplitudes show that a decrease in spreading rate from 7 cm yr to 2 6 cm yr full spreading rate occurred 2 6 ma prior to ridge extinction our data analysis is unable to determine the exact spreading rate of the osbourn spreading center prior to the slowing event the temporal constraints provided by our data show that the osbourn spreading center ceased spreading prior to 87 ma or 93 ma depending on whether the manihiki and hikurangi plateaus rifted at 115 ma or 121 ma our model resolves the conflict between regional models of osbourn spreading with models based on trough characteristics by showing that spreading at the osbourn spreading center was decoupled from pacific phoenix spreading

seismic imaging of hotspot related crustal underplating beneath the marquesas islands volcanism in active continental rift zones1 2or on rifted continental margins3is frequently associated with unusually high lower crustal seismic velocities indicating the presence of large igneous intrusions at the base of the crust the only comprehensive investigation of crustal underplating at an oceanic hotspot beneath hawaii4 6 has yielded controversial results7 here we report the results of seismic refraction experiments across the marquesas islands hotspot trace which show that the island chain is underlain by crust 15 17 km thick and that a large lower crustal region 275 km wide and 2 8 km thick has seismic velocities 7 3 7 75 km s 1 indicative of crustal underplating wide angle reflections occur near the base of the normal lower crustal velocities and at the base of the underplating complex we interpret these reflectors as the relict pre hotspot moho and the current post hotspot moho respectively the high velocity material may be purely intrusive or may consist of a mixture of intrusive and preexisting rocks the volume of the high velocity body is impressive amounting to nearly twice the volume of volcanics erupted at the surface

navigational infrastructure at the east pacific rise 9 50 n area following the 2005 2006 eruption seafloor benchmarks and near bottom multibeam surveys 1 four seafloor benchmarks were deployed with rov jason2 at frequently visited areas along the northern east pacific rise nepr ridge crest near 9 50 n within the ridge2000 epr integrated study site iss bull s eye when used in concert with established deep ocean acoustic positioning techniques these benchmarks provide navigational infrastructure to facilitate the integration of near bottom data at this site by allowing efficient and quantitative coregistration of data and observations collected on multiple dives and over multiple cruises high resolution near bottom multibeam bathymetric surveys also were conducted along and across the ridge crest to provide a morphological and geological context for the benchmark areas we describe the navigation and data processing techniques used to constrain the benchmark positions and outline operational details to effectively use benchmarks at this and other deep ocean sites where multidisciplinary time series studies are conducted the well constrained positions of the benchmarks provide a consistent geospatial framework that can be used to limit navigational uncertainties during seafloor sampling and mapping programs and enable accurate spatial coregistration and integration of observations these data are important to test a range of multidisciplinary hypotheses that seek to link geological chemical and biological processes associated with crustal accretion and energy transfer from the mantle to the hydrosphere at mid ocean ridges

effective elastic thickness of zealandia and its implications for lithospheric deformation abstract zealandia was once located at the active gondwana margin and experienced complicated tectonic evolution from continental breakup to subduction and oblique slipping along the plate boundary here we investigate the lithospheric thermal and rheological properties via spatial variations in effective elastic thickness te using the topography sediment thickness and gravity data the fan wavelet coherence method is employed to recover te values the effects of dynamic topography and gravity in the subduction zone on the estimated te are considered and removed the final results reveal that the te variation pattern generally coincides well with the tectonic regimes relatively high te values up to 22 km are observed in north island and the southernmost part of south island while central and northern south island are dominated by smaller te values 15 km similar to those of the surrounding submerged plateaus we interpret the relatively high te values as indicating the combined lithospheric strength due to the contacting plates in the subduction zone to the north along the tonga kermadec subduction zone the lithosphere surprisingly exhibits anomalously high te values ranging from 22 to 36 km which contradicts the geological evidence of normal faults developed in the incoming pacific plate and the magmatism in the overlying australian plate that generally contribute to low lithospheric strength we speculate that the high te zone is related to the large bathymetry and gravity anomalies in the bending plate and controlled by non isostatic factors finally we find widespread low te values covering the entire arcs and oceanic basins in northern zealandia which may indicate that regional lithospheric thinning associated with delamination weakened the integrated strength of the lithosphere

structure and dynamics of sheared mantle plumes 1 an extensive series of laboratory experiments is used to investigate the behavior of sheared thermal plumes the plumes are generated by heating a small circular plate on the base of a cylindrical tank filled with viscous fluid and then sheared by rotating a horizontal lid at the fluid surface the motion of passive tracers in the plumes is visualized by the release of several dye streams on the hot plate we systematically examine the dependence of the convective flow on four dimensionless numbers a velocity ratio a rayleigh number the viscosity ratio and an aspect ratio we identify and delineate two transitions in the convective behavior from a regime where the plume can spread upstream against the shear to a regime where the entire plume is advected downstream and from a regime of negligible cross stream circulation to a regime with significant cross stream circulation and thermal entrainment our analysis of the steady profiles of the plumes shows that they initially rise with a constant vertical rise velocity this rise velocity depends on the buoyancy flux and ambient viscosity but is almost independent of the centerline plume viscosity which suggests that most of the thermal plume has a viscosity that is much closer to the ambient viscosity than the centerline viscosity as the plumes approach the lid they decelerate as the viscous drag on them steadily increases the lateral spreading of the plumes under the lid is found to be well described by similarity solutions derived for the spreading of compositional plumes on a rigid surface if the effective viscosity of the thermal plumes is taken to be the ambient viscosity rather than the centerline viscosity a similar theoretical model is found to roughly predict the upstream spreading of thermal plumes at low shear but it breaks down at moderate to high shear where the entire plumes are advected downstream when our results are applied to the earth we find that mantle plumes are mostly divided into only two flow regimes in the upper mantle plumes under slow moving plates experience upstream flow and negligible cross stream circulation while plumes under faster moving plates including all pacific plumes experience significant cross stream circulation and are advected downstream we also demonstrate that geochemical heterogeneities in a plume s source region will result in an azimuthally zoned plume and in an asymmetric geographical distribution of geochemical heterogeneities in the erupted hot spot basalts as is seen in the hawaiian galapagos marquesas and tahiti society island chains for individual mantle plumes we determine their diameter and vertical rise velocity as well as the extent of upstream spreading and the rate of lateral spreading under the lithosphere

correction to history of the cretaceous osbourn spreading center 1 in the paper history of the cretaceous osbourn spreading center by nathan j downey joann m stock robert w clayton and steven c cande journal of geophysical research 112 b04102 doi 10 1029 2006jb004550 2007 the tectonic evolution of the osbourn spreading center was determined based on ship track geophysical measurements this paper presents a history in which spreading at the osbourn spreading center began near the beginning of chron c34 121 or 115 ma the average spreading rate at the center was greater than 7 cm a 1 full spreading rate until spreading slowed shortly before the osbourn spreading center became inactive however the age estimates of the extinction of the osbourn spreading center as well as the time before extinction of the change in spreading rate are erroneous due to the confusion of full spreading rates with half spreading rates in the age calculations thus the time intervals in the work of downey et al 2007 were too short by a factor of two 2 as discussed in the work of downey et al 2007 the temporal constraints on the events outlined are relatively poor therefore the major conclusions of the paper concerning the geometry of spreading at the osbourn spreading center remain unchanged however the following modifications apply 3 1 the slowing of the osbourn spreading center occurred 4 11 million years prior to extinction rather than 2 6 million years prior to extinction as stated in paragraphs 47 55 and 58 4 2 the discussion and calculations presented in the first half of paragraph 57 concerning the extinction age of the osbourn spreading center are incorrect spreading at the osbourn spreading center could have ceased during chron c34 after 93 or 87 ma the incorrect extinction ages presented in paragraph 59 and the abstract should be disregarded 5 we thank bruce luyendyk for pointing out the error in our original calculations journal of geophysical research vol 113 b09102 doi 10 1029 2008jb006012 2008 click here for full article

a reexamination of the flexural deformation beneath the hawaiian islands seismically derived depth estimates to the top of the oceanic crust beneath the hawaiian islands indicate that the curvature of the deflected lithosphere is much larger than commonly believed the conservative and model independent curvature estimates exceed 10 7 m 1 and are comparable in magnitude to curvatures at trenches and outer rise systems the depth estimates are used to constrain both two dimensional 2 d and three dimensional 3 d flexural models the curvature constraints require a 2 d variable elastic thickness that decreases from 35 km in areas away from the volcanic load to 25 km directly beneath the load in an attempt to understand the nature of the yielding beneath the hawaiian islands we introduce two new 3 d models the first model combines a realistic yield strength based rheology with a new technique for 3 d flexure calculations in which the elastic plate thickness is curvature dependent the new variable rigidity model predicts an undeformed mechanical plate thickness of 44 km decreasing to 33 km beneath the big island of hawaii the best fitting mechanical thickness corresponds approximately to the depth to the 600 c isotherm in 90 m y old lithosphere the second model uses a broken plate but here the crack is oriented along the weak molokai fracture zone rather than along the island chain trend this unconventional flexure model can explain the observed asymmetry in the depth data across the fracture zone without requiring the excessively large elastic thickness of more conventional broken plate models both the proposed models imply that modeling with constant thickness plates may underestimate the true mechanical plate thickness by being unduly influenced by the weak zone beneath the seamounts

west antarctic peninsula sea ice in 2005 extreme ice compaction and ice edge retreat due to strong anomaly with respect to climate in september october 2005 the juxtaposition of low and high pressure anomalies at 130 degrees w and 60 degrees w respectively created strong and persistent northerly airflow across the west antarctic peninsula wap this had a major impact on regional sea ice conditions with extreme ice compaction in the bellingshausen and east amundsen seas 60 degrees w 130 degrees w but divergence in the west amundsen and east ross seas this resulted in the former in a highly compact marginal ice zone and ice cover mean modeled ice thicknesses of 5 m and an earlier than average maximum extent mid august while rapid ice retreat in late winter spring created a major negative ice extent anomaly compact ice persisted in the subsequent summer other effects were anomalies in air temperature of 1 degrees c to 5 degrees c and precipitation rates to 2 5 mm d the patterns in late 2005 are consistent with the occurrence of a weak la nina and a near neutral southern annular mode with a quasi stationary zonal wave three pattern dominating hemispheric atmospheric circulation once a compact ice edge was created it took only one additional week of strong winds to solidify the pack in place conditions in 2005 are analyzed in the context of 1979 2005 and compared with the springs of 1993 1997 1999 2001 and 2004 a statistically significant increase of the northerly 10 m wind component between 110 degrees w and 125 degrees w occurred in the septembers of 1979 2005 no clear trends occur in other spring months this work underlines the key importance of ice dynamics in recent changes in the wap sea ice regime

implications of a nonlinear 40ar 39ar age progression along the louisville seamount trail for models of fixed and moving hot spots 1 the louisville seamount trail has been recognized as one of the key examples of hot spot volcanism comparable to the classic volcanic hawaiian emperor lineaments the published total fusion 40ar 39ar data of watts et al 1988 showed an astonishing linear age progression firmly establishing louisville as a fixed hot spot in the south pacific mantle we report new 40ar 39ar ages based on high resolution incremental heating 40ar 39ar dating for the same group of samples showing a marked increase in both precision and accuracy one of the key findings in our reexamination is that the age progression is not linear after all the new data show a significantly decreased apparent plate velocity for the louisville seamount trail older than 62 ma but confirm the linear trend between 47 ma and the present day although based on only three samples over 2150 km the most recent volcanic activity in the louisville seamount trail has now been dated at 1 11 0 04 ma for the most southeastern seamount located at 50 26 s and 139 09 w these results indicate that the louisville age progression should be interpreted on the basis of both plate and hot spot motion in this paper we examine our new results in conjunction with the numerical mantle flow models of steinberger et al 2004 that also predict marked deviations from simple linear age progressions with these models we can achieve a good fit to the geometry of both the hawaiian and louisville seamount trails and their age progressions as well as the 15 paleolatitudinal shift observed by tarduno et al 2003 for the hawaiian hot spot between 80 and 47 ma if the model is restricted to pacific hot spots only we can improve the fit to the nonlinear age trend for the louisville seamount trail by allowing an additional rotation change of the pacific plate around 62 ma and by decreasing the initiation age of the louisville plume from 120 to 90 ma this improved model features a significant eastward hot spot motion of 5 between 80 and 30 ma for the louisville hot spot which is quite dissimilar to the southward motion of the hawaiian hot spot during the same time interval followed by a minor 2 latitudinal shift over the last 30 myr if hot spot tracks are considered globally the age trend observed for the oldest part of the louisville seamount trail does not entirely follow the numerical model predictions this may indicate some remaining inaccuracies in the global plate circuit but it may also indicate that the louisville hot spot experienced a motion somewhat different than in the numerical model faster in the interval between 62 and 47 ma but slower before that

global gravity bathymetry and the distribution of submarine volcanism through space and time 1 the seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin relatively few of these features 0 1 however have been dated and so little is known about their tectonic setting one parameter that is sensitive to whether a seamount formed on near or far from a mid ocean ridge is the elastic thickness te which is a proxy for the long term strength of the lithosphere most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of te and then comparing the calculated and observed gravity anomaly the problem with such an approach is that bathymetry data are usually limited to single beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape we therefore use the satellite derived gravity anomaly to predict the bathymetry for different values of te by comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the wessel global seamount database we have obtained 9758 te estimates from a wide range of submarine volcanic features in the pacific indian and atlantic oceans comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate te and its upper and lower bounds te at sites where there is both a sample and crustal age show considerable scatter however and there is no simple relationship between te and age nevertheless we are able to tentatively assign a tectonic setting to each te estimate the most striking results are in the pacific ocean where a broad swath of on ridge volcanism extends from the foundation seamounts and ducie island easter island ridge in the southeast across the equator to the shatsky and hess rises in the northwest interspersed among the on ridge volcanism are flank ridge and off ridge features the indian and atlantic oceans also show a mix of tectonic settings off ridge volcanism dominates in the eastern north atlantic and northeast indian oceans while flank ridge volcanism dominates the northeastern indian and western south atlantic oceans we have been unable to assign the flank ridge and off ridge estimates an age but the on ridge estimates generally reflect we believe the age of the underlying oceanic crust we estimate the volume of on ridge volcanism to be 1 1 106 km3 which implies a mean seamount addition rate of 0 007 km3 yr 1 rates appear to have varied through geological time reaching their peak during the late early cretaceous and then declining to the present day

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

directional median filtering for regional residual separation of bathymetry 1 we present a new spatial filter designed to separate short length scale bathymetric features from regional bathymetry the directional median dim filter divides a given filter circle into n bow tie sectors allocates data points inside the filter circle to each sector based on their relative position within the circle estimates a median for each sector and returns the lowest of these n medians this approach prevents dim filtering from choosing the biased medians near the features on a sloping regional trend which is a serious artifact of standard median filtering as the bathymetry comprises diverse length scale features the separated results vary with the choice of filter width such variations are spatially distributed using a finite range of filter widths we evaluate the spatial distribution of variations in the dim based separation by estimating their median absolute deviation mad at each data point the distribution of mad values is indicative of uncertainties inherent in the separation we demonstrate the effectiveness of dim filtering with both synthetic and actual bathymetric data

experimental volcanology on eruptive products of unzen volcano abstract protracted dome building eruptions may be profitably investigated using laboratory based experiments density distribution studies on the pyroclastic flow deposits of unzen 1990 1995 allow us to apply the results of experimental investigations on unzen samples to the interpretation of the last eruption of unzen here primary laboratory experiments have focused on several aspects of the degassing permeability and the fragmentation behavior threshold speed and efficiency those investigations have been flanked by analyses of flexural strength fracture toughness and seismic velocities to provide new insights into eruption related processes here we present a review of these results and their application to the eruption dynamics of unzen volcano we propose that efforts be made to incorporate routinely such comprehensive experimental analyses into the response to emerging volcanic crises in future

flexure modelling at seamounts with dense cores summary the lithospheric response to seamounts and ocean islands has been successfully described by deformation of an elastic plate induced by a given volcanic load if the shape and mass of a seamount are known the lithospheric flexure due to the seamount is determined by the thickness of an elastic plate te which depends on the load density and the age of the plate at the time of seamount construction we can thus infer important thermomechanical properties of the lithosphere from te estimates at seamounts and their correlation with other geophysical inferences such as cooling of the plate whereas the bathymetry i e shape of a seamount is directly observable the total mass often requires an assumption of the internal seamount structure the conventional approach considers the seamount to have a uniform density e g density of the crust this choice however tends to bias the total mass acting on an elastic plate in this study we will explore a simple approximation to the seamount s internal structure that considers a dense core and a less dense outer edifice although the existence of a core is supported by various gravity and seismic studies the role of such volcanic cores in flexure modelling has not been fully addressed here we present new analytic solutions for plate flexure due to axisymmetric dense core loads and use them to examine the effects of dense cores in flexure calculations for a variety of synthetic cases comparing analytic solutions with and without a core indicates that the flexure model with uniform density underestimates te by at least 25 per cent this bias increases when the uniform density is taken to be equal to the crustal density we also propose a practical application of the dense core model by constructing a uniform density load of same mass as the dense core load this approximation allows us to compute the flexural deflection and gravity anomaly of a seamount in the wavenumber domain and minimize the limitations recognized from the analytic tests then the dense core model is applied to predict the lithospheric flexure beneath howland island in the tokelau seamount chain these results are compared with the predictions of the uniform density model based on age dating of howland and the age of the seafloor traditional te versus age curves predict the elastic plate thickness beneath the seamount to be around 20 km which is comparable to the best dense core model of te 26 km however the best uniform density model is found at te 12 km which is significantly less than the predicted from our investigations of synthetic and real seamount cases we conclude that the dense core model approximates the true mass distribution of a seamount better than the uniform density model finally we suggest that the role of underplating in flexure modelling may need to be reexamined because the dense core model predicts substantially less deflections than the uniform density model without requiring additional buoyancy caused by underplated material

observations of flexure and the rheology of oceanic lithosphere the principal evidence for the long term i e 1 myr mechanical behaviour of the oceanic lithosphere has come from studies of how it deforms in response to large loads such as volcanoes and sediments a model widely used to explain the deformation is an elastic plate in which flexural rigidity depends on the thermal age of the lithosphere at the time of loading an elastic model however is time invariant and does not take into account temporal changes that may occur in the flexural rigidity as a consequence of loading there is evidence for example that the flexural rigidity of the oceanic lithosphere also depends on load age being large at short times and small at long times thus competing effects may exist between thermal cooling which strengthens the lithosphere and some form of load induced stress relaxation which weakens it in order to investigate the relative roles of these processes we have developed a multilayered viscoelastic model which is based on the results of experimental rock mechanics that creep in the lithosphere is a thermally activated process and on a thermal structure that is given by the plate cooling model by comparing the predictions of the model with a new compilation of flexural rigidity estimates we have found that if the upper mantle viscosity is 1020 pa s the activation energy that best describes the long term mechanical behaviour of the oceanic lithosphere is 120 kj mol 1 this parameter pair explains the dependence of flexural rigidity on both plate and load age it also helps account for the subsidence and uplift history of oceanic islands and the stratigraphic patterns that develop in the flexural moats that flank them at atolls a multilayered viscoelastic model explains the rapid subsidence that follows shield building and does not require that the age of the lithosphere that supports a volcano is thermally re set to a younger value our studies suggest that while the oceanic lithosphere has viscoelastic properties the viscosity of its upper layers is so much higher than that of its lower layers that in effect it behaves as a thin elastic plate on long timescales there will therefore be a certain permanence to the observations of oceanic flexure such that they may be used with some confidence to evaluate the tectonic setting of individual features of the seafloor and in some cases their age

rigidity of the atlantic oceanic lithosphere beneath new england seamounts abstract investigation of the flexural response of the oceanic plate subject to loading of new england seamounts provides an insight into the rigidity of the lithosphere fourier domain analysis of the bathymetry and gravity anomalies over new england seamounts suggests that internal density perturbations exist in the crust and or uppermost mantle the loads due to the topography of the seamounts and density perturbations must have been supported by the lithosphere we group the seamounts into three representative ages and adopt a 3 d thin elastic plate model over an invicid mantle to assess the rigidity of the lithosphere under these loads the rigidity values are about 3 7 10 23 n m for lithospheric ages ranging from 30 to 60 my the base of the elastic part of the lithosphere follows an isotherm in the range of 750 850 c suggesting a strong lithosphere that makes the north atlantic ocean difficult to initiate subduction however the thermal blanketing by the thick sediments at scotian basin reduces the rigidity by one to two orders of magnitude and may facilitate initiation of subduction of the north atlantic oceanic lithosphere at scotian margin

crustal intrusion beneath the louisville hotspot track we report here the first detailed 2d tomographic image of the crust and upper mantle structure of a cretaceous seamount that formed during the interaction of the pacific plate and the louisville hotspot results show that at 1 5 km beneath the seamount summit the core of the volcanic edifice appears to be dominantly intrusive with velocities faster than 6 5 km s the edifice overlies both high lower crustal 7 2 7 6 km s and upper mantle 8 3 km s velocities suggesting that ultramafic rocks have been intruded as sills rather than underplated beneath the crust the results suggest that the ratio between the volume of intra crustal magmatic intrusion and extrusive volcanism is as high as 4 5 in addition the inversion of moho reflections shows that the pacific oceanic crust has been flexed downward by up to 2 5 km beneath the seamount the flexure can be explained by an elastic plate model in which the seamount emplaced upon oceanic lithosphere that was 10 myr at the time of loading intra crustal magmatic intrusion may be a feature of hotspot volcanism at young hot oceanic lithosphere whereas magmatic underplating below a pre existing moho may be more likely to occur where a hotspot interacts with oceanic lithosphere that is several tens of millions of years old

modelling of the gravity effects of flexurally compensating thin layers under the afanasy nikitin seamount abstract modelling of the gravity data over the afanasy nikitin seamount is carried out to estimate the variation of the crustal and flexural parameters along the strike of the seamount it is inferred that the compensation of the seamount is due to both the bending of a very thin elastic plate and the occurence of low density material at depth the elastic plate thickness is inferred as 2 km and does not change significantly along the strike of the seamount furthermore the crustal thicknesses range within 12 22 km not showing any systematic variation along the strike of the seamount it is suggested that the afanasy nikitin seamount was emplaced on thin and weak lithosphere very near to a slowly migrating ridge which trapped a hot rising plume resulting in a higher crustal thickness

flexure and gravity anomalies of the oceanic lithosphere beneath the louisville seamount abstract we have calculated the elastic thickness te flexural deflection and gravity anomaly of the oceanic crust beneath the louisville seamount lsc 03 near the kermadec trench a regional residual separation of the bathymetry was performed to remove the effect of other geologic features e g the trench we used the uniform density and dense core models to approximate the total mass of the seamount which was defined as the surface load required for flexural deformation from the flexure modeling results we found that more flexural depression was predicted by the uniform density model than by the dense core model however the uniform density model predicted a significantly smaller gravity anomaly than observed whereas the dense core model minimized the prediction misfits reasonably the best flexure model was found with a te of 16 km for the uniform density model and 6 km for the dense core model the flexure computed with the dense core model was consistent with the seismically detected moho the flexure modeling for lsc 03 thus indicates that the dense core model better approximates the inner structure of the lsc 03 based on the crustal age and geochronology of the given seamount the age of the oceanic crust at the time of seamount formation t is 20 ma if this is the case however the te estimates from both flexure models require some degree of lithospheric reheating by louisville hotspot activity alternatively considering the tectonic plate motion of the osbourn trough t becomes approximately 4 ma this younger lithosphere model is more consistent with the observed flexural deformation and the te estimate from the dense core model therefore the time that the seamount induced lithospheric deformation occurred may be far earlier than the age dated volcanism

anomalous elastic thickness of the oceanic lithosphere in the south central pacific to first order the elastic thickness te of the oceanic lithosphere under mid plate volcanoes increases linearly with the square root of plate age at the time of loading1 4 a large scatter around this simple relationship is observed however suggesting that factors other than age influence the elastic thickness and produce departure from the first order trend5 to look at second order effects we have conducted a worldwide analysis over the pacific atlantic and indian oceans and determined te under sixty volcanoes applying elastic flexure theory and using as observational constraints geoid height data from the seasat satellite we find that for the three main oceans the depth of the elastic layer under volcanoes increases with age of plate at loading time t according to the simple relationship te 2 70 0 15 t1 2 te in km t in myr except under all of the volcanic chains of the south central pacific where this relation does not apply the elastic layer is found to be much thinner there and indicates a broad regional anomaly presumably of thermal origin well correlated with several other geophysical anomalies reported in this region6 10

structural results for la palma island using 3 d gravity inversion 1 a recent gravity survey composed of 317 bench marks all over the island of la palma canary islands is used in combination with satellite data for regional aspects to obtain results about structural properties of the island connected with the tectonic environment and local volcanism to that end a nonlinear three dimensional gravity inversion approach is considered the inversion scheme provides in a nonsubjective form the geometry of the anomalous bodies constructed in a random growth process results from the inversion can be interpreted in the framework of the geologic evolution of this ocean island volcano as a complex composite volcano with a large central body with high density corresponding to the older intrusive part of the basalt complex new unexpected features are enlightened such as large thermal anomalies in the upper mantle southward of la palma as well as fracture en echelon zones associable to a slow active process of dislocation related to the recent volcanism in the southern half of the island the results obtained for la palma as a test site testify to the usefulness of the developed gravity inversion methodology for structural studies on islands in general

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

origin and tectonic evolution of the caroline ridge and the sorol trough western tropical pacific from admittance and a tectonic modeling analysis abstract an independent caroline sea plate surrounded by prominent seismotectonic features is said to exist in the western tropical pacific the caroline sea plate is a transient feature existing only when the caroline philippine sea plate boundary is jumping to the caroline pacific boundary the origin and the tectonic evolution of the caroline ridge sorol trough region which forms the northern part of the caroline pacific plate boundary are examined using a standard admittance analysis a tectonic modeling study and additional direct gravity and magnetic analyses the observed admittance function is fitted well by shallow airy model in the 250 125 km 0 025 k 1 wavelength region whose average crustal thickness t c is 20 km and also by thin plate flexure model of isostasy within the diagnostic waveband of 250 80 km 0 025 k 1 with an effective elastic thickness t e of 5 7 km a simple interpretation of the observed admittance function is that the local airy model of isostasy is the main support mechanism in the study region for the longer wavelength more fundamental topography corresponding to that of the caroline ridge the caroline ridge is inferred to have formed in a near ridge tectonic setting on top of oceanic lithosphere which was young thin and weak at the time of emplacement the tectonic model study after an explanation for rifting along the mendana fracture zone in the nazca plate suggests that the sorol trough formed in the 17 7 ma interval splitting the caroline ridge into two longitudinal halves the rifting along the sorol trough which took place in the manner described in weissel and karner 1989 flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension j geophys res 94 13 919 13 950 necking of unbroken lithosphere post dated the formation of the caroline ridge by 10 15 m y during which time t e in the region increased to 5 10 km the rifting necking along the sorol trough represents a second loading event in the region the combined gravity effects of the rifting as well as the increase in the t e in the region from essentially nil to 5 10 km is reflected in the agreement between the observed admittance and the regional model with a 5 7 km t e in the wavelength region 80 km these results are consistent with the findings and the suggestions of the previous investigators in the region in that 1 the caroline ridge was constructed in the late oligocene by hot spot volcanism on top of oligocene seafloor of the caroline sea back arc basin 2 the eauripik rise genetically related to the caroline ridge formed on young and weak caroline basin lithosphere and is supported by airy manner crustal thickening and that 3 the lithosphere of the caroline ridge was young at the time of rifting necking along the sorol trough on the basis of the current results and the findings of the previous workers a tentative hypothetical rheological model for the crustal density structure of the caroline ridge sorol trough is also given the caroline sea plate appears to be bounded in the north and south by e w trending boundaries that deform diffusely while more normal narrow n s trending boundaries exhibiting more localized albeit slow deformation bound the plate in the east and west

elastic layer thickening with age of the oceanic lithosphere a tool for prediction of the age of volcanoes or oceanic crust summary to first order the estimated thickness te of the elastic part of the oceanic lithosphere increases linearly with the square root of the age of the lithosphere at the time of loading t in order to quantify this relationship in the particular case of volcanic loading a synthesis of te estimates reported both in this paper and in previous studies is conducted excluding the anomalously low estimates from the south central pacific the values are very consistent for the three main oceans and follow the empirical relationship te km 2 7 0 15 t ma the relationship is used to predict the age of volcanoes when the age of the crust is known for the trindade chain in the south atlantic ocean and to predict the age of the crust when the age of the volcanoes is known for the pacific plate east of the tonga trench the age estimates are in good agreement with the structural setting

perturbation to the lithosphere along the hotspot track of la reunion from an offshore onshore seismic transect a 250 km long ne sw lithospheric transect spanning the 40 km wide island of la reunion and its submarine edifice is derived from lines of air gun shots at sea on either side along the assumed hotspot trace seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers providing a system of reversed and overlapping observations low seismic velocity and hence density is found on average for the whole edifice above the oceanic plate we attribute high velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island crossing segment unexpectedly the main seismic interfaces top and bottom of the prevolcanic crust do not show significant flexural downwarping under the island in addition clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle this lithospheric structure provides the first example where underplating occur s beneath an active volcanic island suggesting a genetic relationship the underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity in the plate hotspot model commonly assumed the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material