variations in magmatic processes along the east greenland volcanic margin summary seismic velocities and the associated thicknesses of rifted and igneous crust provide key constraints on the rifting history the differentiation between non volcanic and volcanic rifted margins the driving force of magmatism at volcanic margins that is active or passive upwelling and the temperature anomaly in the lithosphere this paper presents two new wide angle seismic transects of the east greenland margin and combines the velocity models with a compilation of 30 wide angle seismic velocity models from several publications along the entire east greenland margin compiled maps show the depth to basement depth to moho crustal thickness and thickness of high velocity lower crust hvlc with velocities above 7 0 km s 1 first we present two new wide angle seismic transects which contribute to the compilation at the northeast greenland margin and over the oceanic crust between shannon island and the greenland fracture zone velocity models produced by ray tracing result in total traveltime rms misfits of 100 120 milliseconds and 2 values of 3 7 and 2 3 for the northern and southern profiles with respect to the data quality and structural complexity 2 d gravity modelling is used to verify the structural and lithologic constraints the northernmost profile awi 20030200 reveals a magma starved break up and a rapidly thinning oceanic crust until magnetic anomaly c21 47 1 ma the southern seismic transect awi 20030300 exhibits a positive velocity anomaly associated with the shannon high and a basin of up to 15 km depth beneath flood basalts between shannon island and the continent ocean boundary break up is associated with minor crustal thickening and a rapidly decreasing thickness of oceanic crust out to anomaly c21 the continental region is proposed to be only sparsely penetrated by volcanism and not underplated by magmatic material at all compared to the vast amount of magmatism further south break up is proposed to have occurred at the seaward boundaries of the continent ocean transition zones at between 50 and 54 ma propagating from north to south based on a joint analysis incorporating transects from the kejser franz joseph fjord and godthab gulf secondly the variation of the hvlc along the east greenland margin from 60 to 77 n and from transects of its conjugate margin shows inverted emplacement of prominent landward and seaward hvlc thickness portions from north to south in a distribution chart the differences in the hvlc distribution are attributed to one or more of the following three models in the first model it is inferred that a transfer zone detachment acts as a barrier to northward magma flow in the second model underplating results in thicker and highly intruded lower crust with several small scale feeder dykes that locally increase the lower crustal velocities in the third model a second magmatic event associated with the separation of the jan mayen microcontinent is considered lithospheric scale inhomogeneities might be responsible for the heterogeneous melt generation the inversion of the hvlc distribution in continental and oceanic domains and differences in its velocities

variable crustal structure along the juan de fuca ridge influence of on axis hot spots and absolute plate motions multichannel seismic and bathymetric data from the juan de fuca ridge jdfr provide constraints on axial and ridge flank structure for the past 4 8 ma within three spreading corridors crossing cleft northern symmetric and endeavour segments along axis data reveal south to north gradients in seafloor relief and presence and depth of the crustal magma lens which indicate a warmer axial regime to the south both on a regional scale and within individual segments for young crust cross axis lines reveal differences between segments in moho two way traveltimes of 200 300 ms which indicate 0 5 1 km thicker crust at endeavour and cleft compared to northern symmetric moho traveltime anomalies extend beyond the 5 15 km wide axial high and coincide with distinct plateaus 32 and 40 km wide and 200 400 m high found at both segments on older crust moho traveltimes are similar for all three segments 2100 100 ms indicating little difference in average crustal production prior to 0 6 and 0 7 ma the presence of broad axis centered bathymetric plateau with thickened crust at cleft and endeavour segments is attributed to recent initiation of ridge axis centered melt anomalies associated with the cobb hot spot and the heckle melt anomaly increased melt supply at cleft segment upon initiation of axial volcano and southward propagation of endeavour segment during the brunhes point to rapid southward directed along axis channeling of melt anomalies linked to these hot spots preferential southward flow of the cobb and heckle melt anomalies and the regional scale south to north gradients in ridge structure along the jdfr may reflect influence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution

channeling of plume flow beneath mid ocean ridges we investigate the interaction of mantle plumes with mid ocean ridges in 3 d numerical convection models using a finite volume multigrid technique with local grid refinement we are able to incorporate a much stronger temperature dependence of viscosity than was possible in previous models this allows lateral viscosity variations of more than six orders of magnitude which is realistic in a ridge situation in models of ridge centered plumes with constant flux we determine how the shape and the along axis width of the plume head depends on various model parameters the plume material is significantly channeled along the axis of a slow spreading ridge if the plume viscosity is in the order of 1017 pa s the plume material spreads radially below the ridge if the spreading rate is fast or the viscosity contrast between plume and ambient mantle is small in time dependent calculations we investigate the temporal evolution of short term variations of the plume flux at slow spreading ridges these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate our results support the hypothesis that pulsations of the iceland plume are the cause for v shaped topography and gravity anomalies at the reykjanes ridge

observations and numerical modelling of mountain waves over the southern alps of new zealand a detailed case study of mountain waves observed over the south island of new zealand during the southern alps experiment is presented the purpose of the study is to document the waves generated over this region to explore the extent to which high resolution radiosonde data can be used to determine gravity wave parameters and to estimate the vertical flux of horizontal momentum produced by flow over the southern alps organized wave clouds were observed in a visible satellite image on 20 october 1996 and are thought to have been produced by partially trapped lee waves using background flow fields derived from a radiosonde sounding the theoretical horizontal wavelength of the partially trapped mode is calculated and agrees well with that measured from the satellite image a technique called the horizontal projection method is developed to analyse radiosonde soundings for mountain waves specifically the technique determines the dominant horizontal wavelength over the entire depth of a radiosonde sounding this method uses a coordinate transformation to take account of vertical changes in the background flow and hence vertical variations in the vertical wave number as well as changes in the path of the radiosonde this method is applied to a radiosonde sounding to identify the dominant wave mode and emphasizes that radiosonde soundings should not in general be treated as instantaneous vertical profiles when analysing mountain waves the mountain waves observed are simulated using a high resolution numerical model and the results are used to check the consistency of the analysis the numerical model produces a horizontally averaged value of momentum flux in the lower stratosphere of 0 02 n m 2 although this value is probably underestimated by at least a factor of four

slow ridge hotspot interactions from global gravity seismic tomography and 87sr 86sr isotope data summary among the mantle hotspots present under oceanic areas a large number are located on or close to active oceanic ridges this is especially true in the slow spreading atlantic and indian oceans the recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models and allow one to study interactions between mantle plumes and active slow spreading ridges the observed correlations allow us to draw preliminary conclusions on the general links between surficial processes which shape the detailed morphology of the ridge axes and deeper processes active in the upper mantle below the ridge axial domains as a whole the interactions are first studied at the scale of the atlantic the mid atlantic ridge from iceland to bouvet island from the correlation between the zero age free air gravity anomaly which reflects the zero age depth of the ridge axis and sr isotopic ratios of ridge axis basalts the study is then extended to a more global scale the slow ridges from iceland to the gulf of aden by including geoid and uppermantle tomography models the interactions appear complex ranging from the evect of large and very productive plumes almost totally overprinting the long wavelength segmentation pattern of the ridge to that of weaker hotspots barely marking some of the observables in the ridge axial domain intermediate cases are observed in which hotspots of medium activity or whose activity has gradually decreased located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle such observations tend to preclude the use of a single hotspot ridge interaction model and stress the need for additional observations in various plume ridge configurations

thermomechanical erosion of the lithosphere by mantle plumes a simplified semianalytic theory is presented that describes the process of lithosphere thinning through thermomechanical erosion by a hot mantle upwelling or plume the theory compares well with published numerical models but predicts that the rate of thinning is very dependent on the lateral scale of the upwelling and on the viscosity through the lithosphere asthenosphere transition a plume tail is likely to cause rapid local thinning reducing the lithosphere thickness substantially within a few million years plume heads or broadly spreading plume tail material are unlikely to cause significant thinning by this mechanism alone these results are consistent with geophysical evidence for localized thinning under the hawaiian volcanic chain but not under the broader hawaiian swell they are also qualitatively consistent with the distribution of basaltic volcanism in oceanic and continental hotspot tracks the theory predicts that thin continental lithosphere may be further thinned sufficiently by a plume tail to cause crustal melting within about 20 m y thus this mechanism may account for some delayed silicic magmatism on some slow continental plates higher thinning rates would have occurred in the archean

time dependence of mantle plumes some simple theory dynamical calculations indicate that plate tectonics are likely to have a major effect on the initiation of plumes from the basal boundary layer in the mantle and their subsequent evolution plume start because the boundary layer is hotter than the overlying mantle and hence unstable to perturbations the growth rate of perturbations is too weakly dependent on wavelength to control the geographic distribution of plumes instead the distribution of plumes is strongly influenced by the shape of large perturbations associated with slabs and surface plates plumes evolve because downward return flow is needed to replace the material tapped from the boundary layer this flow downwarps the top of the boundary layer several hundred kilometers from the plume and thus restricts further flow of hot material toward an active plume the geometry of this downwarp is strongly influenced by the initial relief of the top of the boundary layer and hence by plate processes the actual history of plates and slabs is thus needed in numerical models for comparison with real data on plume fluxes

simulation of extreme new zealand precipitation events part i sensitivity to orography and resolution abstract the extreme precipitation events with peak observed rainfall of greater than 700 mm over the south island of new zealand were simulated using the dar hydrostatic mesoscale model nested within the ecmwf analyses the ecmwf analyses for two of the events showed a low level jet with mixing ratios greater than 12 g kg 1 crossing the south island of new zealand during the heavy precipitation near a cold front the third case which had smaller mixing ratios occurred as a low level jet and crossed the south island while a low redeveloped downstream three different orographies were used with the 30 km horizontal resolution model runs with progressively increased terrain heights the highest orography was created by artificially inserting the effective barrier of the southern alps to northwesterly flow in the model grid orography had a strong influence on the amount of precipitation the peak precipitation was related to orographic slope while the area averaged precipitation was related to the maximu

the fluid dynamics of plume ridge and plume plate interactions an experimental investigation abstract a number of hotspots occur at or near mid ocean ridges e g iceland causing elevated bathymetry gravity anomalies and geochemical enrichment for hundreds of kilometers along otherwise normal ridge axes this is commonly presumed to be the result of hot plume material from the deep mantle flowing outward from a narrow plume conduit here we explore the dynmics of plume ridge and plume plate interaction using laboratory experiments we model chemically buoyant plumes as opposed to thermally buoyant plumes so we ignore the effects of thermal diffusion while concentrating on the strictly fluid dynamical aspects of the interaction we find that the spreading of a continuously fed plume beneath a fixed plate is described by a simple expression derived from a balance between plume buoyancy and viscous resistance to spreading for plumes spreading beneath ridges we find two remarkably distinct phases of flow at first the plume spreads approximately as it would beneath a fixed plate then a rapid transition occurs to a steady state condition in which the plume width along the ridge waist is constant this width is found to be proportional to a natural length scale that increases with the square root of the plume volumetric flux and the ratio of the mantle plume viscosities and decreases with the square root of the plate spreading velocity although our experimental conditions with uniform mantle viscosity and no thermal diffusion are perhaps oversimplified compared to the real earth the results scale reasonably well to observation our model suggests that sufficient time has elapsed 60 myr for the iceland plume to reach steady state in its interaction with the mid atlantic ridge using independent estimates for the plume volumetric flux at the iceland hotspot and assuming a viscosity ratio of one the model predicts a minimum waist width of about 850 km in close agreement with the observed value of about 920 km data from other hotspots jan mayen azores galapagos are also consistent with the experimental results for average mantle plume viscosity ratios of 1 3 using estimated plume fluxes with a simple conservation of mass equation yields reasonable estimates of plume thickness ranging from 70 to 180 km for these hotspots thus the experimental model appears to be reasonably consistent with observation given that thermal diffusion has been disregarded

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

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

observations of geometry and ages constrain relative motion of hawaii and louisville plumes abstract the classic view of linear island chains as volcanic expressions of interactions between changing plate tectonic motions and fixed mantle plumes has come under renewed scrutiny in particular observed paleolatitudes from the emperor seamounts imply that the hawaii hotspot was 5 15 further north during formation of these seamounts and that rapid retardation of its southward migration was the primary agent forming the angular hawaii emperor bend supporting this view are predictions from fluid dynamic experiments that suggest the general mantle circulation may displace narrow mantle plumes consequently the surface locations of hotspots are not fixed and may have varied considerably in the past however the locations and ages of available rock samples place fundamental limits on the relative motion between the hawaii and louisville hotspots here we use such data to estimate empirical age progression curves for separate chains and calculate the continuous variations in hotspot separations through time while the data are sparse the inferred inter hotspot motion for ages 55 myr appears significant but the observed relative motion is only about half of what is predicted by mantle dynamics models to reconcile the observed paleolatitudes with our observed relative motion requires either a larger contemporaneous southward motion of the louisville hotspot than previously suggested or a moderate component of true polar wander

two station measurements of rayleigh wave group velocity along the hawai ian swell two station measurements of rayleigh wave group velocity between midway atoll and o ahu provide a useful constraint on lithospheric thickness along the hawai ian swell comparison of the observed dispersion curve with age dependent regionalized dispersion curves suggests that the swell acts as 50 110 m y old lithosphere and not as 20 50 m y old lithosphere as is predicted by the currently favored hypothesis for the swell s formation a preliminary isotropic shear velocity model for the swell suggests that the lithosphere may be as thick as 100 km this value is difficult to reconcile with the lithosphere being substantially thinned during its passage over the hawai ian hotspot

compensation of swells and plateaus in the north pacific no direct evidence for mantle convection at intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation a low geoid topography ratio 8 m km in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection after subtracting a reference geoid from the observed geoid previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000 4000 km since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid topography ratios 10 20 m km they were believed to reflect the planform of mantle convection we show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10 since the geoid spectrum is red the residual geoid is dominated by degree 11 when the harmonics of the reference geoid are rolled off gradually the regular pattern of geoid highs and lows disappears in the northeast pacific the new residual geoid reflects the lithosphere age offsets across the major fracture zones in the northwest pacific the residual geoid corresponds to isostatically compensated swells and plateaus we have calculated the geoid topography ratio for 10 swells and plateaus and have found a range of compensation depths the highest geoidaopography ratio of 5 5 m km occurs on the flanks of the hawaiian swell intermediate ratios occur in four areas including the midway swell these intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of airy compensated seamounts low geoid topography ratios occur over the remaining five areas e g emperor seamounts reflecting the absence of a thermal swell our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid we see only indirect evidence of thermal plumes reheating the lower lithosphere

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

history of rift propagation and magnetization intensity for the cocos nazca sspreading center analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the cocos nazca spreading center forward models of the magnetic anomalies accounting for spreading rate variations ridge jumps asymmetric spreading magnetization intensity variations and bathymetry show excellent agreement with observed anomalies spreading rates can be constrained to a common finite rotation history through anomaly 4a with three changes in rates rate changes at about 1 5 ma and 4 1 ma correspond to changes in rate gradients and occur during the well calibrated part of the reversal timescale so they can unquestionably be identified as true changes in plate motion a 15 rate decrease at about 5 2 ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale the change at 4 1 ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0 6 ma rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis propagation has been predominantly away from the hotspot with jumps predominantly south ward propagation rates have ranged from 30 to 120 mm yr commonly near 70 mm yr origin of most propagation sequences is difficult to interpret but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot magnetic anomaly amplitude appears to be a reliable tracer of fe content of lavas several generalizations can be drawn about along axis variations in magnetization intensities since 8 ma high magnetizations are only observed at the far ends relative to the galapagos hotspot of segments at least 150 km long offset at the end of a high magnetization segment is at least 15 km and there are no offsets larger than 30 45 km between high magnetization segments and the reconstructed position of the hotspot we interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along axis flow of evolved magma the gradients in supply result from subaxial flow of hotspot derived asthenosphere in a narrow conduit this flow is only partly obstructed by an offset of 20 30 km but entirely blocked by an offset of 50 km

oceanic spreading center hotspot interactions constraints from along isochron bathymetric and gravity anomalies we analyzed bathymetric and gravity anomalies along present and paleoaxes of oceanic spreading centers influenced by the iceland azores galapagos tristan and easter hotspots residual bathymetry up to 4 7 km and mantle bouguer gravity up to 340 km anomalies are maximum at on axis hotspots and decrease with increasing ridge hotspot separation distance d until becoming insignificant at d 500 km along isochron widths of bathymetric anomalies up to 2700 km depend inversely on paleo spreading rate reflecting the extent to which plume material will flow along axis before being swept away by the spreading lithosphere flux balance arguments suggest that the five hotspots feed material to ridges with comparable fluxes of 2 2 106 km3 m y assuming that the amplitudes of these geophysical anomalies reflect temperature dependent crustal thickness and mantle density variations we suggest that ridge temperature anomalies are maximum 150 225 c when plumes are ridge centered and decrease with increasing ridge hotspot distance due to cooling of the ridgeward migrating plume material

kaapvaal craton changing first and second order controls on sea level from c 3 0 ga to 2 0 ga abstract well preserved supracrustal basin fills including the oldest large depository witwatersrand known makes the kaapvaal craton an ideal example for studying controls on first and second order stratigraphic cyclicity the duration of the three basins studied witwatersrand ventersdorp and transvaal from c 3 0 to c 2 0 ga is almost twice that of the entire phanerozoic period from which the paradigms of classical sequence stratigraphy have been derived amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through earth history this does not appear to be borne out by the precambrian record studied here instead a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first and second orders is preferred first order cycles within the c 3 0 2 0 ga period on kaapvaal are synonymous with the entire evolution of a supracrustal basin fill defined by a specific basin type second order cycles define basin fill packages of unique character which reflect large scale shifts in the accommodation sedimentation balance early to middle archaean evolution of the kaapvaal craton encompassed amalgamation of a mosaic of subdomains to produce an early south southeastern nucleus by c 3 1 ga accretion of composite terranes from both north and west followed from c 3 0 2 7 ga this was contemporaneous with evolution of the c 3 0 2 8 ga witwatersrand basin the high grade limpopo terrane was amalgamated with the kaapvaal craton between 2 95 and 2 69 ga although a younger age for this orogensis is also possible the witwatersrand first order depositional cycle was controlled largely by subduction related tectonic loading and had a duration of c 190 my the c 5 my first order cycle of the c 2 7 ga ventersdorp basin was essentially controlled by thermal uplift induced extensional subsidence and that of the c 650 my transvaal basin by extensional and subsequent thermal subsidence it is noteworthy in the three basins examined that first order cycles controlled by plate tectonics witwatersrand and transvaal basins were about two orders of magnitude longer relative to the ventersdorp plume tectonic cycle second order cycles also varied greatly from c 100 my to c 1 my within the witwatersrand and transvaal basins subsidence allowed extensive seaways to develop within which classical sequence stratigraphic systems tracts can be recognized reflecting different stages of shoreline shifts within the short lived ventersdorp continental basin system sedimentological criteria can be used to discriminate high and low accommodation systems tracts the transvaal first order cycle was terminated at c 2 0 ga by the bushveld complex plume event

mechanical erosion and reheating of the lithosphere a numerical model for hotspot swells it is currently debated if either thermal erosion of the lithosphere or dynamical support is the source of topography and geoid anomalies the origin of tins controversy lies probably in the difficulty to model simultaneously these two effects for this purpose we have studied the time dependent behavior of two dimensional convection with a temperature and pressure dependent viscosity the use of a control volume method allows us to define a rigid zone simulating the mechanical lithosphere the interface between the lithosphere and the convective mantle is determined by a viscosity cutoff first some experiments model the rise of a plume below the lithosphere in order to observe the evolution of the uplift and thus to appreciate the various processes involved in the swell formation before the plume reaches the base of the thermal lithosphere an uplift a few hundred meters in amplitude develops which can only be ascribed to a pure dynamical support the major uplift occurs when the ductile part of the lithosphere the convective boundary layer is squeezed by the plume the reheating of the mechanical lithosphere takes place after this transient stage of dynamical erosion however this late process is very slow but can magnify the amplitude of the swell if the lithosphere stays long enough above the plume these results shed some light on the different mechanisms occurring during the swell formation but the configuration modeled does not correspond to the one expected for actual hotspot swells they feature plume rising up to the lithosphere while natural situations correspond to lithosphere drifting above preexisting plumes an experiment with a moving lithosphere was run and shows that thermal erosion does not affect significantly a moving lithosphere even for relatively slow drifting velocities few centimeters year indeed the thermal structure of the lithosphere is not modified above the 800 c isotherm except for a motionless plate in this case the resulting swell should be greater this could explain why azores crozet or cap verde swells are so high on the other hand the shape of a swell over a moving lithosphere is strikingly reminiscent of the hawaiian swell

regional uplift associated with continental large igneous provinces the roles of mantle plumes and the lithosphere the timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes here we review geological indicators of surface uplift associated with five continent based magmatic provinces emeishan traps 260 million years ago ma siberian traps 251 ma deccan traps 65 ma north atlantic phase 1 61 ma and phase 2 55 ma and yellowstone 16 ma to recent all five magmatic provinces were associated with surface uplift surface uplift can be measured directly from sedimentary indicators of sea level in the north atlantic and from geomorpholocial indicators of relative uplift and tilting in yellowstone in the other provinces surface uplift is inferred from the record of erosion in the deccan north atlantic and emeishan provinces transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation observations of the spatial and temporal relationships between surface uplift rifting and magmatism are also important in constraining models of lip formation onset of surface uplift preceded magmatism in all five of the provinces biostratigraphic constraints on timing of uplift and erosion are best for the north atlantic and emeishan provinces where the time interval between significant uplift and first magmatism is less than 1 million years and 2 5 million years respectively rifting post dates the earliest magmatism in the case of the north atlantic phase 1 and possibly in the case of siberia the relative age of onset of offshore rifting is not well constrained for the deccan and the importance of rifting in controlling magmatism is disputed in the emeishan and yellowstone provinces in these examples rifting is not a requirement for onset of lip magmatism but melting rates are significantly increased when rifting occurs models that attempt to explain emplacement of these five lips without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift rifting and magmatism for example small scale convection related to craton or rift boundaries edge driven convection cannot easily explain widespread 1000 km scale transient surface uplift emeishan deccan north atlantic and upper mantle convection initiated by differential incubation beneath cratons the hotcell

asthenosphere flow model of hotspot ridge interactions a comparison of iceland and kerguelen we develop a numerical model to test the asthenosphere flow paradigm in which hotspots feed the low viscosity asthenosphere and lithosphere growth consumes the asthenosphere the dynamics of this flow model are controlled by the relative position of the hotspot to the ridge absolute plate velocities lithosphere and asthenosphere rheologies and tectonic boundary conditions the model is applied to two distinct regions the iceland hotspot centered on the mid atlantic ridge and the kerguelen hotspot located near the southeast indian ridge the iceland model generates along ridge flow rates as high as 30 cm y for an asthenosphere viscosity of 5 1018 pa s these flow rates are consistent with previous interpretations of the origin of southward pointing v s south of iceland the preferred kerguelen model produces significant hotspot to ridge flow followed by along ridge flow velocities from the off axis hotspot to the ridge are 66 cm y for a model within which asthenosphere viscosity varies from 5 1018 to 5 1019 pa s the asthenosphere flow paradigm can explain major features of hotspot ridge interactions for both on axis and off axis hotspots

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

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

the bent hawaiian emperor hotspot track inheriting the mantle wind bends in volcanic hotspot lineaments best represented by the large elbow in the hawaiian emperor chain were thought to directly record changes in plate motion several lines of geophysical inquiry now suggest that a change in the locus of upwelling in the mantle induced by mantle dynamics causes bends in hotspot tracks inverse modeling suggests that although deep flow near the core mantle boundary may have played a role in the hawaiian emperor bend capture of a plume by a ridge followed by changes in sub pacific mantle flow can better explain the observations thus hotspot tracks can reveal patterns of past mantle circulation

crustal thickness along the western galapagos spreading center and the compensation of the galapagos hotspot swell wide angle refraction and multichannel reflection seismic data show that oceanic crust along the galapagos spreading center gsc between 97 w and 91 25pw thickens by 2 3 km as the galapagos plume is approached from the west this crustal thickening can account for v52 of the 700 m amplitude of the galapagos swell after correcting for changes in crustal thickness the residual mantle bouguer gravity anomaly associated with the galapagos swell shows a minimum of 325 mgal near 92 15pw the area where the gsc is intersected by the wolf darwin volcanic lineament wdl the remaining depth and gravity anomalies indicate an eastward reduction of mantle density estimated to be most prominent above a compensation depth of 50 100 km melting calculations assuming adiabatic passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of v30 c the associated thermal expansion and increase in melt depletion reduce mantle densities but to a degree that is insufficient to explain the geophysical observations the largest density anomalies appear at the intersection of the gsc and the wdl our results therefore require the existence of compositionally buoyant mantle beneath the gsc near the galapagos plume possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the galapagos islands that is later transported to the gsc our estimate for the buoyancy flux of the galapagos plume 700 kg s 31 is lower than previous estimates while the total crustal production rate of the galapagos plume 5 5 m 3 s 31 is comparable to that of the icelandic and hawaiian plumes

lithospheric controls on melt production during continental breakup at slow rates of extension application to the north atlantic rifted margins form from extension and breakup of the continental lithosphere if this extension is coeval with a region of hotter lithosphere then it is generally assumed that a volcanic margin would follow here we present the results of numerical simulations of rift margin evolution by extending continental lithosphere above a thermal anomaly we find that unless the lithosphere is thinned prior to the arrival of the thermal anomaly or half spreading rates are more than 50mmyr 1 the lithosphere acts as a lid to the hot material the thermal anomaly cools significantly by conduction before having an effect on decompression melt production if the lithosphere is thinned by the formation of extensional basins then the thermal anomaly advects into the thinned region and leads to enhanced decompression melting in the north atlantic a series of extensional basins off the coast of northwest europe and greenland provide the required thinning this observation suggests that volcanic margins that show slow rates of extension only occur where there is the combination of a thermal anomaly and previous regional thinning of the lithosphere

radar studies of heavy convective rainfall in mountainous terrain heavy rainfall topography storm motion and storm evolution are closely linked for four storms that produced catastrophic flooding along the front range of the rocky mountains and the east slope of the blue ridge mountains storms selected for detailed study in this paper are the rapidan storm of june 27 1995 the fort collins storm of july 28 1997 the buffalo creek storm of july 12 1996 and the monocacy storm of june 18 1996 the buffalo creek storm and the fort collins storm occurred in the front range of the rocky mountains in colorado the rapidan and monocacy storms occurred along the east slopes of the blue ridge of virginia and southern pennsylvania the four storms caused catastrophic flooding at drainage basin scales between 1 and 1000 km 2 the scale of flood response for these events imposes a need to characterize rainfall variability at very fine space scales and timescales of the order of 1 km spatial scale and 1 5 min timescale a fundamental issue for the hydrometeorology of extreme rainfall in mountainous terrain is whether anomalously large rainfall accumulations in orographic convection result from anomalously slow net storm motion anomalously large rainfall rates or both anomalous storm motion and processes resulting in catastrophic rainfall rates are examined for each of the four storms of particular importance for anomalous storm motion in orographic convection are interactions between the low level wind field and terrain features

numerical modeling of chemically buoyant mantle plumes at spreading ridges the geometry of spreading of plumes beneath midoceanic ridges is investigated by three dimensional numerical modeling with the goal of characterizing the width of the plume along the ridge or the waist width chemically buoyant plumes are modeled in order to compare to previously reported laboratory tank experiments the plume is generated near the bottom of the box and rises and forms a mushroom head with some entrainment of surrounding fluid the head flattens at a considerable depth beneath the ridge before rising to the surface once the plume reaches the surface the head is quickly divided by the diverging plates and the waist width is found to have reached a steady state value w the results show that w is proportional to the square root of the volumetric flux of the plume divided by the diverging plate speed which is consistent with previously reported experimental data our scaling law gives an independent method for estimating the volumetric flux of mantle plumes the calculated plume fluxes are two to four times larger than previous estimates if plume buoyancy is purely of thermal origin then excess temperatures can be estimated from the fluxes for iceland azores and the galapagos the calculated excess temperatures are 140 57 and 51 c respectively in agreement with recent independent estimates from modeling of gravity and bathymetry

precipitation sublimation and snow drift in the antarctic peninsula region from a regional atmospheric model 1 a regional atmospheric model with a horizontal grid spacing x of 14 km is used to study the surface mass balance components precipitation sublimation and snow drift in the region of the antarctic peninsula ap an integration is performed for the 7 year period 1987 1993 using a realistic forcing at the lateral model boundaries and at the sea surface output from this integration indicates that the precipitation reaches its maximum value on the northwestern slope of the ap where the upward motion in the atmosphere is largest uplift occurs upstream of the barrier affecting the precipitation distribution over sea the effect of the barrier on the precipitation distribution over the bellingshausen sea might have important implications for the ocean circulation in this region the mean precipitation over the grounded ice of the ap 1 20 m water eq yr 1 is 6 times larger than the mean value over all the grounded ice of antarctica our estimates for the surface sublimation and wind transport of snow over the grounding line toward the sea are 9 and 6 1 of the precipitation respectively in situ data of the wind distribution at three coastal sites located on the northern eastern and western sides of the ap are used to evaluate the modeled wind field which is important for the snow drift calculations for two of the three sites considered the prevailing wind direction and bimodal wind distribution are correctly represented by the model the calculated distribution of accumulation and ablation due to snow drift shows a complex pattern the wind removes snow from the spine of the ap where the near surface flow field diverges whereas deposition occurs mainly on the eastern slopes where the near surface flow field converges an intercomparison between two 7 year integrations at different horizontal resolution x 14 km and x 55 km shows that the precipitation on the northwestern slope is very sensitive to the model resolution in the x 14 km integration precipitation on the northwestern slope is higher than in x 55 km because of higher vertical velocities resulting in a 35 increase in average precipitation over the grounded ice of the ap

microphysical and geometrical controls on the pattern of orographic precipitation patterns of orographic precipitation can vary significantly both in time and space and such variations must ultimately be related to mountain geometry cloud microphysics and synoptic conditions here an extension of the classic upslope model is presented which incorporates an explicit representation in the vertical dimension represents the finite growth time of hydrometeors their downwind advection by the prevailing wind and also allows for evaporation for a simple mountain geometry the authors derive an analytical solution for the precipitation rate which can be understood in terms of four nondimensional parameters the finite growth time and slanting hydrometeor trajectories give rise to some interesting possibilities a precipitation rate that maximizes at intermediate values of the horizontal wind speed localized precipitation efficiencies in excess of 100 and a reverse rain shadow with more precipitation falling on the leeward flank than on the windward flank

observational constraints on models of the hawaiian hot spot swell a detailed analysis of the topography and geoid associated with the hawaii emperor seamount chain reveals that while geoid topography ratios are high over the southeastern part of the chain they fall off with distance away from the current hot spot location the topographic expression of the swell has two maxima near the intersections between the seamount chain and the murray and molokai fracture zones respectively both geoid and topography rapidly decrease westward of the intersection with the murray fracture zone it appears that the region of the pacific plate between the two fracture zones has been more susceptible to reheating and hot spot penetration than elsewhere along the chain alternatively the observed pattern may be indicative of time variation in the strength of the hawaiian hot spot

geochronology of age progressive volcanism of the oregon high lava plains implications for the plume interpretation of yellowstone 1 the high lava plains province hlp is a late cenozoic bimodal volcanic field at the northern margin of the basin and range province in southeastern oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the yellowstone snake river plain ysrp trend we present 40ar 39ar ages for 19 rhyolite domes 5 rhyolite ash flow tuffs and 34 basaltic lavas from the hlp the previously identified trend of westward migration of hlp rhyolites is confirmed the rate of propagation is 33 km m y from 10 to 5 ma slowing to 13 km m y after 5 ma the duration of silicic volcanism at any locus is 2 m y three older hlp dacite domes yielded ages of 15 5 ma basalts are not age progressive we identify several episodes of increased basaltic activity at 7 5 7 8 5 3 5 9 and 2 3 ma with the younger episode likely continuing into the recent the hlp and ysrp trends emerged from the axis of middle miocene basaltic volcanism of the columbia river and steens basalts we propose a model in which 1 miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin 2 the hlp trend is the result of westward flow originating at the craton margin and 3 the ysrp trend is the trace of the motion of the north american plate over the tail of the plume

temporal variation of the hawaiian plume flux abstract the magnitude of the hawaiian swell which surrounds the hawaiian volcanic chain does not decline monotonically and smoothly with distance along the chain the observed undulations are interpreted here as reflecting past variations in the buoyancy flux of the hawaiian plume rather than the monotonic thermal decline of the swell with age as has been inferred previously this interpretation is supported by seismic evidence that indicates there has been little thinning of the lithosphere which it is shown further implies that the time scale for thermal subsidence will be very long the largest change in flux was an increase in the plume flux by more than a factor of two about 25 30 myr ago the size of the volcanic edifices shows a broad correlation with the variations in the swell supporting this interpretation

lateral flow of hot plume material ponded at sublithospheric depths local buoyancy forces drive the flow of ponded plume material along the base of the lithosphere this processes complicates the geological effects of plumes beyond those of simple passage of plates over point heat sources variations in the depth to the base of the lithosphere significantly affect lateral flow the flow rate is modified to a moderate extent by locally thin lithosphere beneath midplate hotspots such as hawaii the effect is relevant when numerical models are rescaled to better fit the observed uplift and the observed distance from the hotspot to the nose of the swell variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes effectively forming an upside down drainage pattern plume material flows upslope toward ridge axes from off axis plumes and along ridge axes from on axis plumes at sufficiently low spreading rates thinner lithosphere exists above plume material than above normal asthenosphere the edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material pressure release melt is associated with the upslope flow of plume material it is expected to cause volcanism away from the plume and to sometimes supply melt depleted plume material to ridge axes that are upslope from the region of voluminous melting

laboratory investigation of the interaction of off axis mantle plumes and spreading centres mantle plumes and mid ocean ridge spreading centres are the dominant phenomena through which mass and heat are transported from the mantle to the earth s surface it now seems that the dispersion of near ridge plumes beneath the lithosphere is modulated strongly by mid ocean ridges in particular geochemical and geophysical observations have suggested that rising plumes are diverted towards and feed nearby ridges 1 7 here we confirm the feasibility of this model with laboratory experiments that incorporate the essential physical and fluid dynamic aspects of a plume ridge upper mantle system our results indicate that an off axis plume may communicate thermally and chemically with a spreading ridge through a narrow sub horizontal conduit instead of a broader radially spreading plume head a necessary condition for this communication is the presence of a lithospheric or rheological boundary layer that thickens away from the ridge axis owing to conductive cooling interestingly we find that for high plume temperatures increasing the plume thermal buoyancy may inhibit rather than enhance plume ridge interaction as a result of increased erosion of the overlying lithosphere

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

the topographic and gravitational expression of density anomalies due to melt extraction in the uppermost oceanic mantle abstract evidence suggests that basaltic extrusion can decrease the density of the depleted source region by up to 2 for 20 partial melting this hypothesis is tested by calculating the effect of the density anomaly between the depleted and undepleted mantle on the topography gravity and geoid fields and comparing the anomalies to the observed topography gravity and geoid anomalies over volcanic features in the oceans if the source region lies at the base of the lithosphere we find that the gravity and geoid anomalies are proportionally more sensitive to the density contrast than the topography anomaly this effect is most pronounced for wavelengths less than 1000 km since mid ocean ridge volcanism the largest source of volcanism in the oceanic plates is fairly uniform along the ridge the depleted material in the oceanic plates and the corresponding topography and gravity anomalies should also be somewhat uniform in extent any long wavelength anomalies will be obscured in the topography and geoid fields by other anomalies at similar wavelengths such as slab and deep mantle anomalies however at mid plate swells and seamounts if the density contrast is greater than 0 4 the topography gravity and geoid anomalies are measurable to be consistent with the wavelengths represented in the geoid and topography data at hawaii the depleted material must be spread into a thin layer by convective flow at mid plate swells even in the thin layer model the topography and geoid anomalies due to the depleted material would still be as large as 200 m and 2 m respectively after the convective flow producing the bulk of the swell is removed the anomaly due to the depleted material would remain and would be apparent in the geoid and topography fields at seamounts if the source region is near the base of the plate then the presence of depleted material should be distinguishable in the geoid field but be somewhat obscured by flexural effects therefore although a topographic or geoid anomaly due to depletion has not been observed the density anomaly cannot be discounted because only careful scrutiny and modelling of mid plate swells and seamounts would distinguish its effect

the dynamics of plume ridge interaction 2 off ridge plumes a variety of geophysical and geochemical evidence indicates that ascending mantle plumes can interact with ocean ridges located up to 1400 km away i investigate the dynamics of this interaction using a simple model in which a point source with volume flux q analogous to a plume stem releases buoyant fluid into a viscous corner flow driven by the divergence of rigid surface plates with thickness kx u 1 2 where u is the half spreading rate the point source is located at a distance xp from the ridge and ridge migration is neglected the buoyant fluid forms a thin sublithospheric layer whose thickness s x y satisfies a nonlinear advection diffusion equation describing the balance of advection by the corner flow buoyancy driven self spreading flow toward the ridge along the sloping base of the lithosphere and continuous accretion into the lithosphere numerical solutions of this equation yield scaling laws for the lateral extent w waist width of plume material along the ridge the fraction r of the plume flux that crosses the ridge and the maximum value of xp beyond which interaction ceases the sloping base of the lithosphere has only a minor few tens of percent influence on these quantities which are determined principally by the balance of advection and self spreading an extension of the model to include plume induced lithospheric thinning shows that this process increases the waist width by an amount of order 10 finally the model provides a new explanation for the observation that plumes interact primarily with ridges that are migrating away from them rather than toward them

crustal thickness and the average depth and degree of melting in fractional melting models of passive flow beneath mid ocean ridges in simple models of passive flow beneath mid ocean ridges fractional melting of the upwelling mantle produces unexpected relationships amongst crustal thickness degree of partial melting and the average depth of melting if the degree of melting is estimated on the basis of basalt composition then the average degree of melting is one third the maximum degree of melting and the average depth of melting is about two thirds the maximum depth in the simplest models the crustal thickness is about 1 5 times the product of the average fraction of melting and the height of the melting column if the average degree of melting represented in mid ocean ridge basalts were 10 and melt production continued to the base of the crust then only a 40 km high residual melting column would be required to produce a 6 km thick crust in more realistic models melting beginning at 60 km below the base of the crust and continuing to the top of the mantle would produce a 6 km thick crust with average degree of melting of 6 67 or melting beginning at 67 km and ending at 30 km below the base of the crust would produce a 6 km thick crust with an average degree of melting of only about 5 4 models of composition produced by fractional melting must consider the rate of melt production and the degree of melting throughout the melting region at the points where the melt is actually produced

south arch volcanic field newly identified young lava flows on the sea floor south of the hawaiian ridge several young lava fields were imaged by gloria sidescan sonar along the hawaiian arch south of hawaii the largest 35 by 50 km across includes a central area characterized by high sonar backscatter and composed of several flow lobes radiating from a vent area reflection profiling and sea floor photography indicate that the central lobes are flat sheet flowsbounded by pillowed margins thin surface sediment and thin palagonite rinds on lava surfaces suggest ages of 1 10 ka vents are localized along the arch crest near bases of cretaceous seamounts two dredged flows are basanite and alkalic basalt broadly similar to rejuvenated stage and some pre shield alkalic lavas on the hawaiian ridge arch volcanism representsperipheral leakage of melt from the hawaiian hot spot over much larger areas than previously recognized

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

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

anomalously young volcanoes on old hot spot traces i geology and petrology of cocos island cocos island is the summit of a seamount on the aseismic cocos ridge a proposed trace of the galapagos hot spot the island lies on a portion of the ridge that is middle miocene in age but k ar and paleomagnetic dates indicate that cocos is only about 2 m y old cocos thus offers a rare opportunity for an on land study of seamount volcanism superimposed on an early hot spot volcanism cocos island was built in three major stages that define three lithostratigraphic units 1 shield building 2 explosive volcanism and 3 post explosive volcanism stages all cocos rocks belong to the typical oceanic island alkali basalt to trachyte series and have fairly homogeneous sr 87 sr 86 sr 0 70299 0 70308 nd 143 nd 144 nd 0 512952 0 513001 and pb 206 pb 204 pb 19 214 19 251 207 pb 204 pb 15 553 15 596 206 pb 204 pb 38 899 39 036 isotopic ratios the cocos rock series was generated by fractional crystallization of olivine clinopyroxene plagioclase ilmenite and apatite from similar alkali basalt parental magmas sr nd and pb isotopic ratios indicate that cocos and galapagos volcanic rocks may have come from a common although heterogeneous mantle reservoir and this implies that the young cocos volcano is still a part of the galapagos hot spot signal