guyot and seamount morphology and tectonics of the hawaiian emperor elbow by multi beam sonar abstract five guyots and five seamounts are shown from the southernmost end of the emperor chain an area called the hawaiian emperor bend or the milwaukee seamount group in the north central pacific ocean the data are from the u s naval oceanographic office navoceano multi beam survey files the bend is shown to occur over an area encompassing 260 km questions are raised as to the relative proximity of different flat top depths and formational possibilities in the light of the differing geomorphology on the features in addition there are two new features taish and go shirakawa seamounts and four additional new names of features are introduced antoku guyot toba guyot and genji and go sanj seamounts

shallow marine phreatomagmatic eruptions through a semi solidified carbonate platform odp leg 144 site 878 early cretaceous mit guyot west pacific abstract leg 144 1995 of the ocean drilling program recovered basalts and volcaniclastic material from the volcanic basement of several guyots in the northwest pacific including mit guyot in site 878 tectonic reconstruction suggests that this seamount originated as an intraplate volcano on the pacific superswell in the early cretaceous tephra emplaced by mass flow within the sedimentary cap is the subject of this study the tephra unit consists of palagonitized moderately to strongly vesicular basaltic clasts of apparent phreatomagmatic origin carbonate mud to grainstone clasts and armoured lapilli armoured lapilli are inferred to have been formed during eruption within a steam envelope that developed above the vent due to magma water interaction and high gas content of the magma another distinctive feature of the tephra unit are abundant truncation surfaces that cut the lithotypes due to subsidence and are juxtaposed to the same lithotypes these truncation surfaces are inferred to represent intra eruption slip surfaces which formed during eruption of the volcano that generated the tephra pile the resulting slumps initiated grainflows and dilute turbidity currents whose deposits are interspersed with those of high concentration density currents a subaqueous setting of deposition and eruption is indicated by the abundance of sideromelane glass shards cauliflower chilled clasts the underlying marine carbonates the lack of tachylite within the deposits and sedimentary structures within the breccia

depth and age in the north pacific the concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer in past studies problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor in this study we evaluate different tectonic histories for the quiet zones and construct a revised isochron chart for the north pacific we check and modify the current sediment thickness charts using new deep sea drilling data and revised ideas concerning reflectors in the southwestern north pacific averaging the synbaps digital data base into half by half degree elements and correcting for sediment load we plot basement depth versus age for the entire north pacific because of the skewness in the data associated with seamounts we present the data as contours about the mode and find that two thirds of the observations lie within 300 m of this value on crust between 0 and 80 ma the ocean floor subsides with the square root of age on ocean floor older than 100 ma the depths in general flatten with age with areally correlateable swells and valleys the hawaiian and line island swells correlate positively and the mariana basin valley negatively with geoid height the flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor further the correlation of the swells and valleys with geoid height presents evidence for organized convection in the upper mantle beneath older lithosphere in the north pacific

insular erosion isostasy and subsidence organic reefs and shore erosion record the intersection of sea level with islands from this record it is possible to reconstruct the history of vertical movement of the islands and the adjacent deep sea floor including midplate swells as judged by coral thickness islands with barrier reefs sink as though they were on thermally youthful crust regardless of the actual age reefless islands do not sink until truncated by erosion apparently thermal subsidence is balanced by isostatic uplift in response to erosion barrier reefs prevent wave erosion of encircled volcanoes and capture products of stream erosion so that isostatic uplift is eliminated insular shelves widen initially at rates of 0 6 to 1 7 kilometers per million years the rates decrease with time thus the subsidence of islands depends on the size of the is land and the presence of reefs and it may not always be the same as that of the surrounding oceanic crust

lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotspots the fate of this material and the thermal structure of the lithosphere depend on the vigor of secondary convection through the steep thermal gradient above the ponded plume material it has been proposed either that 1 secondary convection is so vigorous that material hotter than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger lithosphere or 2 that the plume material retains its excess temperature for a significant time and the lithosphere thins slowly a parameterized method of modeling transient secondary convection is developed to examine this question calculations indicate that secondary convection is likely to be sluggish unless partial melting greatly lowers viscosity in addition vigorous secondary convection would greatly retard upstream asthenospheric flow of plume material beneath the nose of the hawaiian swell and asthenospheric flow of hot material away from hotspots on slowly moving plates like cape verde this conclusion is compatible with limited seismic evidence that indicates that the lithosphere is not detectably thinned beneath the hawaiian swell

mantle temperature anomalies along the past and paleoaxes of the galapagos spreading center as inferred from gravity analyses to better understand the effects of hot spots on mid ocean ridge thermal structure we investigate the subsurface density structure of the galapagos spreading center and nearby lithosphere using shipboard gravity and bathymetry data we obtain maps of mantle bouguer anomalies mba by removing from the free air gravity the attractions of seafloor topography and a 6 km thick model crust comparison of observed and theoretical mba profiles along isochrons for ages 0 0 7 7 ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle this result is consistent with the notion that seafloor topography along the galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs along the ridge axis the mba decreases from the east and west toward the galapagos hot spot by 90 mgal reaching a minimum nearest the hot spot at 91 w seafloor topography mirrors the mba along axis increasing by 1 1 km toward the hot spot these variations in mba and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50 25 c toward the hot spot the predicted crustal thickening of 2 4 km nearest the hot spot accounts for 70 75 of the along axis mba and bathymetry anomalies mantle density variations account for the rest of the anomalies from the crustal isochron of age 7 7 ma to the present day axis the along isochron amplitudes of mba decrease from 150 to 90 mgal the corresponding along isochron bathymetry anomalies decrease from 1 7 to 1 1 km these observations along the paleoaxes of the galapagos spreading center indicate that the axial temperature anomaly was 70 hotter in the past 86 25 c and has steadily decreased to 50 25 c as the ridge axis migrated away from the galapagos hot spot these along isochron temperature anomalies however have remained well below that estimated for the hot spot itself 200 c indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10 20 times greater than that along the ridge axis over the past 7 7 m y

implications of volcano and swell heights for thinning of the lithosphere by hotspots the heights of hotspot volcanoes are modeled by assuming isostatic equilibrium between the magma column and the adjacent lithosphere the depth to the level of compensation is assumed to be related to the thickness of the lithosphere after it has been reheated and thinned by a hotspot there is a square root relationship between volcano height and lithospheric age relationships between volcano height and lithospheric thickness and between reheated thickness and age of the lithosphere can be constrained within the limits imposed by uncertainties in lithospheric and magma densities if the reheated thickness determined from the isostatic model can be compared with lithospheric thickness determined from thermal models of the lithosphere then a relationship between the thickness of the lithosphere before and after reheating can be derived combining the upper limit on the reheated thickness age relationship with a theoretical expression derived by s t crough which relates swell height to lithospheric age reset thickness and physical constants yields a swell height age relationship that is in good agreement with empirical swell height data this agreement supports the assumption that the lithospheric thickness defined isostatically by volcano heights is closely related to the thermal thickness of the lithosphere a result that in turn supports the thermal origin of hotspot swells

rapid formation of the shatsky rise oceanic plateau inferred from its magnetic anomaly shatsky rise in the northwest pacific ocean is probably the oldest extant oceanic plateau and as with most such features its origin is uncertain both oceanic plateaus and continental flood basalts are thought to be formed by rapid voluminous eruptions that occur when the head of a newly born mantle plume ascends to the base of the lithosphere1 3 high eruption rates have been estimated for flood basalts for example 1 5km3 yr 1 for the deccan traps2 from dating of lava flows but the inaccessibility of oceanic plateaus makes it necessary to extrapolate dating information from a small number of samples and sites4 here we estimate the eruption rate of shatsky rise by a method that is indirect but has the virtue of sampling the entire volume of the plateau above the surrounding sea floor the main southern part of the plateau has a positive magnetic anomaly corresponding to a reversed geomagnetic polarity at the time of eruption using age constraints to identify the longest period of reversed polarity during which the plateau could have formed we estimate that 2 106 km3 of material erupted at a minimum rate of 1 7km3 yr 1 this issomewhat less than the rate of 8 22 km3 yr 1 estimated for the ontong java plateau4 but still represents a massive eruption consistent with the plume head hypothesis

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

easterly wave regimes and associated convection over west africa and tropical atlantic results from the ncep ncar and ecmwf reanalyses ncep ncar and ecmwf daily reanalyses are used to investigate the synoptic variability of easterly waves over west africa and tropical atlantic at 700 hpa in northern summer between 1979 1995 1979 1993 for ecmwf spectral analysis of the meridional wind component at 700 hpa highlighted two main periodicity bands between 3 and 5 days and 6 and 9 days the 3 5 day easterly wave regime has already been widely investigated but only on shorter datasets these waves grow both north and south of the african easterly jet aej the two main tracks noted over west africa at 5 n and 15 n converge over the atlantic on latitude 17 5 n these waves are more active in august september than in june july their average wavelength phase speed varies from about 3000 km 8 m s 1 north of the jet to 5000 km 12 m s 1 south of the jet rainfall convection and monsoon flux are significantly modulated by these waves convection in the inter tropical convergence zone itcz being enhanced in the trough and ahead of it with a wide meridional extension compared to the 3 5 day waves the 6 9 day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics the only main track is located north of the aej along 17 5 n both over west africa and the atlantic the mean wavelength is higher about 5000 km long and the average phase speed is about 7 m s 1 then the wind field perturbation is mostly evident at the aej latitude and north of it the perturbation structure is similar to that of 3 5 days in the north except that the more developed circulation centers moving more to the north lead to a large modulation of the jet zonal wind component south of the aej the wind field perturbation is weaker and quite different the zonal wind core of the jet appears to be an almost symmetric axis in the 6 9 day wind field pattern a clockwise circulation north of the aej being associated with a counter clockwise circulation south of the jet and vice versa these 6 9 day easterly waves also affect significantly rainfall convection and monsoon flux but in a different way inducing large zonal convective bands in the itcz mostly in the trough and behind it as opposed to the 3 5 day wave regime these rainfall anomalies are associated with anomalies of opposite sign over the guinea coast and the sahelian regions over the continent these waves are more active in june july and in august september over the ocean gate phase i gave an example of such an active 6 9 day wave pattern considered as a sequence of weak easterly wave activity this phase was also a sequence of high 6 9 day easterly wave activity we suggest that the 6 9 day regime results from an interaction between the 3 5 day easterly wave regime maintained by the barotropic baroclinic instability of the aej and the development of strong anticyclonic circulations north of the jet over west africa and both north and south of the jet over the atlantic significantly affecting the jet zonal wind component the permanent subtropical anticyclones azores libya st helena could help initiation and maintenance of such regime over west africa and tropical atlantic based on an a priori period band criterion our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over west africa and tropical atlantic ncep ncar and ecmwf reanalyses are in good agreement the main difference being a more developed easterly wave activity in the ncep ncar reanalyses especially for the 3 5 day regime over the atlantic

late cretaceous and paleogene tectonic evolution of the north pacific ocean the late cretaceous history of the northern pacific ocean has not been adequately deciphered largely because a major plate reorganization occurred during the cretaceous magnetic quiet interval using primary data to reconstruct plate motions from fracture zone trends and late cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region especially the emperor and chinook troughs the primary event in our reconstruction is the subduction of the old northwest pacific triple junction new relative plate motions imposed by formation of convergent boundaries along both the northern pacific and farallon plates caused the farallon plate to crack this subdivision occurred 82 m y ago and resulted in the formation of the kula and chinook plates the chinook plate was bounded on the north by the chinook kula ridge the western arm of the great magnetic bight on the west by the southern emperor trough a slowly spreading rift valley on the south by the mendocino transform and on the east by the chinook farallon ridge the northern emperor trough formed the initial western boundary of the kula plate from 82 m y ago to 50 m y ago this configuration was stable the chinook plate expanded the chinook kula ridge migrated north and the kula plate was subducted with the subduction of this ridge the chinook plate adhered to the pacific plate and the pacific farallon ridge became the north south trending feature reflected by anomalies 22 and younger this reconstruction obviates the awkward late cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region to achieve closure of vector triangles representing the relative late cretaceous plate motions some extension must occur presumably across the eastern mendocino fracture zone between the northern and southern portions of the farallon plate these two plates apparently were independent units between 105 and 50 m y ago the initiation and cessation of rapid north farallon north american convergence 82 and 50 m y ago may have determined the nature and duration of the laramide orogeny

case study of a developing african easterly wave during namma an energetic point of view abstract the west african perturbation that subsequently evolved into hurricane helene 2006 during nasa s african monsoon multidisciplinary analysis namma 15 august 14 september 2006 and amma s third special observing period sop 3 15 29 september 2006 has been simulated with the nonhydrostatic meso nh model using parameterized convection the simulated disturbance evolved over west africa and the adjacent eastern tropical atlantic through interactions between different processes at the convective scale mesoscale and synoptic scale the aim of this paper is to quantify the energetics of the simulated disturbance a set of energy equations is first developed in the hydrostatic case to solve the limitations of lorenz s analysis when applied to a finite domain it is shown that this approach is also valid in the compressible and in the anelastic case in order to apply it to the meso nh results application to the simulated pre helene disturbance allows one to determine the most important terms in

evidence of two regimes of easterly waves over west africa and the tropical atlantic synoptic scale easterly waves at 700 hpa have been studied over west africa and the tropical atlantic for the summers of 1979 1995 spectral analyses fast fourier and wavelet transforms of 700 hpa meridional wind component along 17 5 n enable to point out two band periods between 3 and 5 days and between 6 and 9 days an example of each wave is shown composite analysis confirms the evidence of two such regimes of easterly waves during summer over west africa and the tropical atlantic the 6 9 day wave regime differ from the 3 5 day wave regime by larger anticyclonic cells originated from the libyan and the azores areas and by more northern tracks

evolution of hawaiian basalts a hotspot melting model melt generation and extraction along the hawaiian volcanic chain should be largely controlled by the thermal structure of the hawaiian swell and the heat source underneath it we simulate numerically the time and space dependent evolution of hawaiian volcanism in the framework of thermal evolution of the hawaiian swell constrained by residual topography geoid anomalies and anomalous heat flow along the hawaiian volcanic chain the transient heat transfer problem with melting relationships and variable boundary conditions is solved in cylindrical coordinates using a finite difference method the model requires the lithosphere to be thinned mechanically by mantle plume flow melting starts quickly near the base of the plate when the hotspot is encountered thermal perturbation and partial melting are largely concentrated in the region where the original lithosphere is thinned and replaced by the mantle flow the pre shield loihi alkalic and tholeiitic basalts are from similar sources which are a mixture of at least three mantle components the mantle plume asthenosphere and the lower lithosphere the degree of partial melting averages 10 20 with a peak value of 30 near the plume center as a result of continuous compaction melts are extracted from an active partial melting zone of about 10 20 km thickness which moves upwards and laterally as the heating and compaction proceed the rate of melt extraction from the swell increases rapidly to a maximum value of 1 105 km3 m y over the center of the heat source corresponding to eruption of large amounts of tholeiitic lavas during the shield building stage this volume rate is adequate to account for the observed thickness of the hawaiian volcanic ridge melts from direct partial melting of the mantle plume at depth may be important or even dominant at this stage although the amount is uncertain at the waning stage mixing of melts from the mantle flow pattern with those from low degree partial melting of the lithosphere may produce postshield alkalic basalts after the plate moves off the heat source continuous conductive heating can cause very low degree partial melting less than 1 of the lithosphere at shallow depths for about one million years this process may be responsible for producing post erosional alkalic basalts the extraction time for removing such small amount of melts is about 0 4 2 m y similar to the time gap between the eruption of post erosional alkalic lavas and the shield building stage our results show that multi stage hawaiian volcanism and the general geochemical characteristics of hawaiian basalts can be explained by a model of plume plate interaction

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

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

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

two cretaceous volcanic episodes in the western pacific ocean stratigraphic analyses of cores recovered by the deep sea drilling project from the western pacific ocean confirm that cretaceous volcanic activity in that region was common and that two separate major episodes of activity can be distinguished the older episode aptian to cenomanian in age resulted in the formation of most of the oceanic plateaus in the western pacific in the younger santonian to maastrichtian episode several island and seamount chains formed especially those in the west central pacific that trend north northwest these volcanic events among the most extensive in the marine geologic record covered an area of at least 30 10 6 km 2 possibly the older episode was responsible for a cretaceous sea level high and concomitant epi continental transgression the younger episode of volcanism certainly covered an extensive area but its effect upon sea level may not have been as great as that of the older event

african easterly jet barotropic instability waves and cyclogenesis abstract this study investigates the structure of the african easterly jet focusing on instability processes on a seasonal and subseasonal scale with the goal of identifying features that could provide increased predictability of atlantic tropical cyclogenesis the modern era retrospective analysis for research and applications merra is used as the main investigating tool merra is compared with other reanalyses datasets from major operational centers around the world and was found to describe very effectively the circulation over the african monsoon region in particular a comparison with precipitation datasets from the global precipitation climatology project shows that merra realistically reproduces seasonal precipitation over that region the verification of the generalized kuo barotropic instability condition computed from seasonal means is found to have the interesting property of defining well the location where observed tropical storms are detected this property does not appear to be an arti

dominant influence of volcanic loading on vertical motions of the hawaiian islands abstract rates of island vertical motion above intra plate hotspots record the processes that arise from interactions between lithospheric plates and mantle plumes to assess the contribution of mantle and lithospheric processes to surface motion above the hawaiian hotspot we compare simple models of lithospheric deformation to vertical motion rates measured from dated paleoshorelines and tide gauge records in the hawaiian islands our analysis shows that observed uplift and subsidence rates mainly record the flexural response of the lithosphere to volcanic loads the effective elastic plate thickness that best fits the spatial distribution of subsidence and uplift rates is 40 km consistent with previous estimates based on total vertical deflection because volcanic loading dominates the vertical motion signal hawaiian islands appear to follow a predictable trajectory of vertical motion when they reside within one flexural half wavelength of the active volcanic center islands initially subside at rapid and decreasing rates in the first 1 myr following their construction uplift relatively slowly 1 2 5 myr following their construction and eventually subside again but at slow rates within 5 myr of their construction this observed pattern of uplift and subsidence is consistent with the pattern of vertical motion predicted to result from volcanic loading at the hawaiian hotspot lithospheric migration over the long wavelength topographic swell associated with the hawaiian hotspot has a comparatively minor influence on island uplift and subsidence its contribution to island vertical motion is not readily observed in our data with the possible exception of some uplift observed in the past 500 kyr on o ahu that might correspond to non steady state behavior of the hawaiian plume

geologic evolution of hess rise central north pacific ocean cores from four deep sea drilling project dsdp sites 310 464 465 and 466 and seismic reflection profiles provide data that are used to interpret the geological evolution and paleoenvironments of hess rise a prominent oceanic plateau in the central north pacific ocean hess rise apparently formed in the southern hemisphere along the western flank of the pacific farallon ridge 110 to 100 m y b p core stratigraphies and lithologies show the response of sedimentation to subsidence and northward movement of hess rise on the pacific plate oceanic islands which crowned hess rise during its early evolution were eroded and subsequently subsided below sea level major structural trends include three northwest trending 327 arms or ridges and an east northeast trending southern hess rise that parallels the mendocino fracture zone 060 normal faults offset basement as much as 3 000 m along the southern edge and 1 500 m on the western flank of hess rise many faults were active during sedimentation tholeiitic basalt from the base of hole 464 trachyte from the base of hole 465a and alkalic basalt clasts within sediment of hole 466 show the diversity of rock types that constitute the igneous basement a major rock unit is middle cretaceous limestone chalk and minor chert that form the basal sedimentary unit some limestone samples rich in organic carbon reflect accumulation above the carbonate compensation depth ccd within a mid water oxygen minimum zone the organic carbon rich sediments probably were deposited on the submarine slopes of islands and banks that were at upper bathyal depths as hess rise crossed the wide equatorial divergence where increased upwelling and biogenic productivity contributed to high accumulation rates the source of organic matter was mostly lipid rich autochthonous marine organic matter analyses of sediment samples from across the cretaceous tertiary boundary at site 465 show that there was a significant decrease in surface water temperature and biological productivity an abrupt increase in transition metals and iridium suggests that an outside source perhaps extraterrestrial was the cause for many of the sudden oceanographic geochemical and biological changes at the boundary

cretaceous to recent record of elevated 3he 4he along the hawaiian emperor volcanic chain helium isotopes are a robust geochemical tracer of a primordial mantle component in hot spot volcanism the high 3he 4he up to 35 ra where ra is the atmospheric 3he 4he ratio of 1 39 10 6 of some hawaiian island volcanism is perhaps the classic example new results for picrites and basalts from the hawaiian emperor seamount chain indicate that the hot spot has produced high 3he 4he lavas for at least the last 76 million years picrites erupted at 76 ma have 3he 4he 10 14 ra which is at the lower end of the range for the hawaiian islands but still above the range of modern mid ocean ridge basalt morb 6 10 ra this was at a time when hot spot volcanism was occurring on thin lithosphere close to a spreading ridge and producing lava compositions otherwise nearly indistinguishable from morb after the hot spot and spreading center diverged during the late cretaceous the hot spot produced lavas with significantly higher 3he 4he up to 24 ra although 3he 4he ratios stabilized at relatively high values by 65 ma other chemical characteristics such as la yb and 87sr 86sr did not reach and stabilize at hawaiian island like values until 45 ma our limited 3he 4he record for the hawaiian hot spot shows a poor correlation with plume flux estimates calculated from bathymetry and residual gravity anomalies van ark and lin 2004 if 3he is a proxy for the quantity of primordial mantle material within the plume then the lack of correlation between 3he 4he and calculated plume flux suggests that variation in primordial mantle flux is not the primary factor controlling total plume flux

convection of north pacific deep water during the early cenozoic the history of deep water formation and abyssal flow is poorly known but important to establish in order to develop a better understanding of changes in oceanic mass heat salt and nutrient transport north atlantic high latitude regions currently are the dominant deep water producers but paleogeographic constraints proxy interpretations and physical models have suggested other modes for the past such as those characterized by high latitude pacific sources subtropical sources or widespread nonlocalized sources here we present new north pacific late cretaceous paleogene nd isotope data from fossil fish debris and detrital silicates combined with results of coupled climate model simulations to test these hypothesized circulation modes the data and model simulations support a circulation mode characterized by high latitude bipolar pacific convection deep convection in the north pacific and likely the south pacific was most intense during the relatively cool portion of the late cretaceous paleocene and waned prior to the peak global warmth of the early eocene ca 52 ma

the west african monsoon dynamics part iii the quasi biweekly zonal dipole abstract this paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the african monsoon during northern summer here identified as the quasi biweekly zonal dipole qbzd the qbzd is primarily characterized by a quasi stationary zonal dipole of convection whose dimension is larger than the west african monsoon domain with its two poles centered along the guinean coast and between 30 and 60 w in the equatorial atlantic the qbzd dynamical processes within the atlantic africa domain are examined in some detail the qbzd has a dipole pattern associated with a walker type circulation in the near equatorial zonal plane it is controlled both by equatorial atmospheric dynamics through a kelvin wave like disturbance propagating eastward between its two poles and by land surface processes over africa inducing combined fluctuations in surface temperatures surface pressure and low level zonal winds off the coast of west africa when convection is at a min

depth and geoid anomalies over oceanic hotspot swells a global survey the broad depth and geoid anomalies associated with 23 hotspot swells in oceanic areas have been analyzed maximum height and geographical extent of the topographic swell and of the geoid anomaly as well have been measured for each hotspot the results indicate a clear increase of the topographic swell height with age of the underlying lithosphere from values in the range 300 500 m at young ages to values in the range 1 500 2 000 m at ages larger than 100 ma the geoid anomaly amplitude also increases with plate age from nearly zero close to mid ocean ridges to 6 8 m over old plates on the other hand the geographical extent of the swell does not show any clear relationship with plate age the mean lateral extent of swells range from 1 000 to 1 500 km swells located close to spreading ridges show a significant non zero depth anomaly but are associated with negligible geoid signal these results complete those of a previous study where the apparent compensation depth of oceanic hotspot swells was showed to increase linearly with the square root of plate age and coincide roughly with the base of the thermal lithosphere this trend may either be interpreted in terms ofmore lithospheric thinning or dynamical support besides both seem necessary to explain the observed bathymetry in proportion evolving with aging of the lithosphere less

on the relationship between african easterly waves and the african easterly jet this idealized modeling study investigates how convectively triggered african easterly waves aews are influenced by the intraseasonal variability of the african easterly jet aej a set of 10 day averaged zonally varying basic states is constructed with the ncep 2 reanalysis 1979 2006 a primitive equation model is used to simulate linear aews on each of these basic states using the same idealized convective heating localized over the darfur mountains as an initial trigger it is shown that the transient response depends strongly on the basic state with the same trigger many configurations of the aej fail to produce a wave disturbance while others produce strong easterly wave structures necessary conditions for the development of strong waves can be characterized by a strong jet a strong vertical shear or a strong and extended potential vorticity reversal in strong wave cases the jet is extended to the south and west and the jet core is aligned with the maximum of surface westerlies maximizing the vertical shear the pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the aej revealed by an empirical orthogonal function eof analysis of the set of basic states

origin of intraplate volcanoes from guyot heights and oceanic paleodepth the height of a guyot as measured from the surrounding regional sea floor to the vol cano s slope break records the water depth at the time the guyot submerged thus guyot heights may be used as indicators of the paleodepth of the surrounding ocean floor we compile data on the heights of 68 intraplate guyots and atolls in the pacific ocean as well as 46 volcanic islands in the pacific atlantic and indian oceans we find that guyot heights generally increase with the age of the lithosphere upon which they werc cmplaced although therc is a large amount of scattcr in nearly all cases seamount height and thus seafloor paleodepth is less than expzcted of normal seafloor these results suggest that most of the volcanoes in this study formed on anomalously shallow seafloor consistent with formation at hotspots to characterize thermal anomalies associ ated with these hotspot swells we model guyot heights by calculating the isostatic uplift predicted for normal lithosphere that has been partly reheated and is underlain by anomalously hot mantle this model is able to explain the anomalous water depth at most of the seamounts with hotspot thermal anomalies of 100 30o0c the heights of a few volcanic chains however are not anomalously low suggesting that these volcanoes are not associated with hotspots in addition the observed trend of hawaiian emperor guyot heights as well as the subdued morphology and gravity signature of the oldest emperor seamounts supports our hypothesis that cretaceous age meiji seamount may have formed on or near a spreading center

posterosional volcanism in the cretaceous part of the hawaiian hotspot trail multibeam bathymetry and seismic reflection profiling shows the cretaceous products of the hawaiian hotspot to be a ridge of coalesced guyots that prolong the n nw trend of the more isolated paleocene emperor seamounts a 10 000 km2 plateau detroit plateau where the trail changes strike to nw and the obruchev rise hotspot ridge which extends to the kamchatka trench the northernmost guyots were submerged and tilted southeast by the load of new shield volcanoes added to the end of the chain then secondary volcanism built small cones on their summit platforms and in a gap between two guyots dredged rock samples and the distribution of the cones indicates that on one guyot these submarine eruptions were in the alkalic postshield stage of hawaiian volcanism and at another were probably in the alkalic rejuvenated stage seamount building eruptions at detroit plateau produced lavas nepheline melilitites that belong geochemically to the alkalic rejuvenated stage and are very similar to pleistocene lavas in the hawaiian islands however these eruptions postdated passage off the hotspot plume by a much longer time than the 0 5 2 5 m y observed in the hawaiian islands and were probably initiated by different tectonic processes

the middle triassic evolution of the bangong nujiang tethyan ocean evidence from analyses of oib type basalts and oib derived phonolites in northern tibet in this paper we present new major and trace element chemical data for the basalts and phonolites of the nare ocean island fragment naoi as well as zircon u pb age data and hf isotope compositions for the naoi phonolites in the middle segment of the bangong nujiang suture zone northern tibet our aim is to assess the genesis of these rocks and to reconstruct the middle triassic evolution of the bangong nujiang tethyan ocean bnto the naoi retains an ocean island type double layered structure comprising a basaltic basement and an oceanic sedimentary cover sequence conglomerate and limestone the latter accompanied by layers of erupted phonolite near the top of the sequence the basalts in the naoi are enriched in light rare earth elements and high field strength elements nb ta zr hf and ti and they exhibit chondrite normalized ree patterns and primitive mantle normalized trace element patterns similar to those of ocean island basalts taking into consideration their high dy yb sm yb and la sm ratios we conclude that the naoi basalts were derived from the partial melting of garnet peridotite in the mantle the naoi phonolites have lree enriched chondrite normalized ree patterns with negative eu anomalies eu eu 0 41 0 43 and primitive mantle normalized trace element patterns with enrichments in nb ta zr and hf and depletions in ba u sr p and ti given the high contents of nb 172 256 ppm ta 11 8 16 0 ppm zr 927 1117 ppm and hf 20 8 26 9 ppm and the very low contents of mgo 0 11 0 25 wt the very low mg values 5 10 and the near zero contents of cr 1 27 7 59 ppm ni 0 43 7 19 ppm and co 0 11 0 38 ppm and the small and homogeneously positive e hf t values 4 9 to 9 5 we infer that the naoi phonolites were formed by the fractional crystallization of an oib derived mafic parent magma the phonolites of the naoi contain zircons that yielded u pb ages of 239 and 242 ma indicating that the naoi formed during the middle triassic these data combined with data from modern ocean islands e g canary islands cape verde fernando de noronha tristan da cunha and gough in the atlantic ocean and society and austral cook in the pacific ocean lead us to infer that the bnto was open for a long time before the middle triassic and that the ocean had already developed into a mature ocean with a thick oceanic lithosphere by at least the middle triassic

ninigi and godaigo seamounts twins of the emperor chain by multi beam sonar abstract bathymetry is shown for two heretofore unknown seamounts in the emperor chain ninigi and godaigo seamounts they were discovered by the u s naval oceanographic office and named by the author very accurate bathymetry and positioning allows a clarification of previous mid chain bathymetric charts and an explanation of hot spot multiple conduit flow

benham rise unveiled morphology and structure of an eocene large igneous province in the west philippine basin abstract the benham rise is an oceanic large igneous province at the western margin of the philippine sea it has an ocean island basalt geochemistry a surface area of 0 11 mkm2 and a volume of 0 13 mkm3 a crustal thickness of 15 km is estimated from gravity anomalies volcanism was most active in the early stages of its formation during the lutetian from 47 9 ma to 41 3 ma although volcanic activity extended to 26 ma multibeam bathymetry with almost 100 coverage has revealed the morphology of benham rise for the first time it consists of a main body 310 km by 330 km with the narra loro and molave spurs extending 100 200 km from its eastern side the main body is built on a shield platform from 5200 m to 3800 m sub sea the platform flanks consist of 3 15 km wide terraces with scarps as high as 100 m to 300 m the platform is surmounted by a crest exhibiting caldera morphology at an average depth of 2500 m the crest is named apolaki caldera and may be the world s largest known caldera with a diameter of 150 km features like a breached rim intra caldera benches and a resurgent dome indicate a multi phase volcanic history consisting of both quiet and explosive eruptions the bathymetry and publicly available geological and geophysical data record three main stages in the tectonic history of benham rise shield building caldera formation and post caldera late stage volcanism the platform base is interpreted to have formed over a hotspot beneath the central basin spreading center during the shield building phase between 47 9 ma to 42 5 ma the arrowhead shaped molave spur is interpreted to have formed late in this stage along a ridge that propagated away from the hotspot the caldera formation followed and is interpreted to have occurred before 41 3 41 5 ma when benham rise was still joined with urdaneta plateau the narra spur was formed by late stage hotspot related volcanism as benham rise and urdaneta plateau separated and ended with the eruptions of the vinogradov seamount at 26 ma

heat flow and the thermal origin of hot spot swells the hawaiian swell revisited we present 150 new heat flow measurements obtained at eight sites along a 1230 km long profile across the hawaiian swell about 700 km ese of midway island most of the measurements include in situ thermal conductivity determinations which helped to reduce the statistical uncertainties 95 confidence at all sites to 59 mw m 2 about 20 higher than the predicted heat flow for 100 ma seafloor based on simple lithospheric cooling models thus even though there is a small increase in heat flow with age along the swell when compared with the off swell heat flux the anomalous heat flow associated with the hawaiian swell is probably of the order of 5 10 mw m 2 and arguably may not exist at all a previous investigation von herzen et al 1982 may have overestimated the magnitude of the heat flow anomaly associated with the hawaiian swell by comparing heat flux measurements on the swell with values expected for simple lithospheric cooling models the heat flow anomalies associated with the bermuda rise and the cape verde rise may also be smaller than previously estimated given the uncertainties in the heat flux on the normal seafloor surrounding these swells the lack of a significant heat flow anomaly associated with the bathymetric expression of the hawaiian swell is inconsistent with simple models of lithospheric reheating dynamic support accompanied by modest temperature increases 80 ma oceanic lithosphere

provenance of sediment in the labrador current a record of hinterland glaciation over the past 125 ka the labrador current flows southward along the continental margin off eastern canada and is the principal source of sediment to the outer continental margin areas where ice sheets did not cross the continental shelf we present multi proxy mineralogical and geochemical records based principally on x ray diffraction and x ray fluorescence for core 2011031 059 from northern flemish cap to determine changes in supply and hence to reconstruct provenance evolution during the last glacial cycle the calcite to dolomite ratios in heinrich layers ranged from 2 to 4 suggesting carbonate layers derived from hudson strait as early as marine isotope stage mis 5a d mineral abundance shows three time dependent predominant sediment sources other than in heinrich layers i baffin bay shelves and adjacent land in mis 5 and 1 with high ca sr kaolinite and feldspars and low clay minerals and calcite dolomite ratios ii labrador in mis 3 with low clays and grey color and iii north east newfoundland shelf in mis 2 and 4 with higher concentrations of clay minerals red sandstones and transition elements these results allow a reconstruction of principal ice streams supplying the north west atlantic over the last full glacial cycle

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