transitions in axial morphology along the southeast indian ridge shipboard bathymetric and magnetic profiles across the southeast indian ridge seir were analyzed in order to examine the nature of along axis variations in axial morphology at this intermediate spreading rate ridge three types of axial morphology are observed along the seir an axial high a shallow 200 700 m deep axial valley and a deep 1000 m deep axial valley an axial high is found to the east of the australian antarctic discordance aad east of 128 e and between 82 e and 104 e a shallow rift valley is found from 104 e to 114 e and from 82 e westward past the amerstdam st paul hotspot asp to about 30 s 75 e deep rift valleys are found from 114 e to 128 e in the vicinity of the aad and from the indian ocean triple junction iotj at 25 s 70 e to about 30 s 75 e the transition near 30 s occurs in an area of constant zero age depth and does not appear to result from an increase in mantle temperature it could be the result of the rapid increase in spreading rate along that portion of the seir the most likely cause of the other transitions in axial morphology is variations in mantle temperature the transitions between the different types of axial morphology are well defined and occur over a limited distance transitions in axial morphology are accompanied by significant changes in ridge flank topographic roughness the transitions from axial valleys to axial highs are also accompanied by changes in the amplitude of the seafloor magnetic anomalies our observations suggest that there are distinct modes rather than a continuum of axial morphology on the seir and that there appears to be a threshold mechanism for a rapid change between different states of axial morphology the asp has only a limited influence on the seir the ridge axis is marked by an axial valley for the entire distance from the iotj up to and past the asp the ridge axis becomes shallower as the asp is approached from the northwest but only by about 300 m over a distance of 800 km in addition the ridge continues to become shallower away from amsterdam island toward the transition to an axial high at 82 e 350 km to the east of the asp the kerguelen hotspot appears to exert a major influence on the morphology of the seir by feeding asthenospheric material to the ridge axis a long narrow finger like gravity high extends ene away from the kerguelen plateau for a distance of 500 km shipboard data show that the gravity high results from a large volcanic ridge the ridge appears analogous to the rodriguez ridge extending from the reunion hotspot toward the central indian ridge a series of lower and broader lineated gravity highs extend from the volcanic ridge toward the seir in the ridge segment between the 81 e and 85 e transforms which is the westernmost segment with an axial high the only region of significant off ridge seismicity on the antarctic flank of the seir is a diffuse band of epicenters extending from kerguelen to the seir within the segment between the 81 e and 85 e fracture zones the along axis gradient in depth from 86 e to the aad and the transitions in axial morphology at 104 e and 114 e most likely reflect along axis variations in mantle temperature and melt production rate due to distance from the kerguelen hotspot and the influence of the aad

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

helium isotopes on the pacific antarctic ridge 52 5 41 5 s 1 the first isotopic data and concentrations of helium are reported for the pacific antarctic ridge between 52 5 s and 41 5 s the 4he 3he ratio is extremely homogeneous over more than 1200 km with a mean ratio of 99 275 r ra 7 29 and a standard deviation of 2719 0 19 which is the lowest dispersion observed for the global mid oceanic ridge system moreover the menard t f is a frontier between two mantles with slightly different helium isotopic ratios 96 595 1520 and 100 347 2330 no difference in the helium concentration between the two ridge segments defined by the menard t f can be observed as well as no significant difference in the u and th contents suggesting that the difference in helium isotopic ratio is old 500 my and may represent a slight difference in degassing or and trace element depletion history

interactions between ridges and plumes in the last 30 years geologists have made much progress on understanding slabs lithosphere and mid ocean ridges but mantle plumes remain enigmatic in their perspective montagner and ritsema 1 chart recent progress largely based on seismic anisotropy toward understanding how such plumes interact with mid ocean ridges existing data are insufficient however to determine whether mantle plumes act independently from plate tectonics the authors call for ocean bottom seismic stations to settle this question 1 http www sciencemag org cgi content full 294 5546 1472

the deep earthquakes of 1921 1922 in northern peru abstract we present a detailed analysis of the 1921 and 1922 deep events in northern peru which we identify as the only reliably located earthquakes in the vicinity of the great 1970 deep colombian shock all other reported seismicity in a three dimensional radius of 550 km centered on the 1970 hypocenter is erroneous and can be relocated to other seismic provinces the 1921 1922 earthquakes relocate approximately 230 km to the south of the 1970 event at depths of 630 km and 660 km respectively a combination of centroid moment tensor inversion and forward modeling using ray theory and reflectivity synthetics yields focal mechanisms indicating down dip compression and seismic moments of 1 2 1027 dyn cm and 6 1027 dyn cm respectively the 1921 1922 and 1970 events take place on a piece of subducted farallon plate aged approximately 55 ma i e anomalously young to support deep seismicity we suggest that this unusual seismic activity may be due to a local heterogeneity on the slab whose nature however remains unknown

effects of ridge geometry on mantle dynamics in an oceanic triple junction region implications for the azores plateau abstract plate boundary geometry can affect the nature of magmatism along a mid ocean ridge the azores plateau is located in a complex geological setting that includes a triple junction tj an oblique and recently formed ultra slow spreading ridge a zone of diffuse seafloor deformation a major fracture zone and a postulated hotspot the precise character of the hotspot is somewhat debated as some lines of evidence indicate it may not be a classic deep seated plume however seismic and gravity data suggest plateau crustal thicknesses of 8 km or more implying some mechanism for excess melting to assess the role of ridge geometry in creating the azores plateau this study uses a finite element numerical model to isolate the effects of selected aspects of plate boundary configuration on mantle flow and melt production in a tj kinematically similar to the azores tj the model focuses on the slowest spreading ridge in the tj analogous to the terceira rift the effect of the varying ridge obliquity observed along the terceira rift is also assessed using an independent 1 d melting model in general relatively little melt production is predicted along the terceira rift analogue except for regions closest to the tj where the proximity of a faster spreading ridge increases temperatures within the melting zone in the 1 d melting model with mantle temperatures of 1350 c melt thicknesses of 2 km are calculated for the least oblique segments while more oblique segments produce little to no melt the presence of a long discontinuity simulating the gloria fz has little effect on mantle dynamics for axial distances

initiation of a plume ridge interaction in the south pacific recorded by high precision pb isotopes along hollister ridge 1 the southern pacific ocean offers the rare possibility to study a situation where a spreading ridge the pacific antarctic ridge par migrates toward a fixed hot spot the louisville hot spot small 1995 hollister ridge is a 450 km long linear structure whose position between the par axis and the most recent edifices of the louisville hot spot trail led some authors to suggest that the ridge is genetically related to the hot spot small 1995 wessel and kroenke 1997 mapping and sampling of the ridge in 1996 revealed however that the contribution of the louisville plume material to its mantle source is minor and suggested that it might be the result of intraplate deformation geli et al 1998 vlastelic et al 1998 we report new highly precise pb isotopic data from hollister ridge which 1 confirm that the maximal contribution of the louisville plume in the centrally volcanic active part of the ridge probably does not exceed 20 15 and 35 for lower and upper limits and 2 reveal through time an increasing plume influence the initiation of the louisville plume involvement in the source of hollister ridge is estimated to have occurred between 1 04 and 0 77 myr ago it thus followed closely the most recent volcanic activity reported along the louisville trail 1 11 ma koppers et al 2004 this suggests that hollister ridge has recorded the dispersion of the louisville plume as the spreading ridge approached the hot spot assuming that the louisville hot spot is located near the youngest seamount dredged along the louisville seamount chain hollister ridge lies along the shortest path of pressure release connecting the hot spot to the spreading axis this path involves first an abrupt upwelling across the eltanin fault system and subsequently a more progressive migration toward the spreading axis because hollister ridge is older than 2 5 ma the structure might not be the consequence of the plume ridge flow instead hollister ridge most likely emplaced through a lithospheric crack geli et al 1998 which subsequently may have captured the plume ridge flow

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

geochemistry of axial seamount lavas magmatic relationship between the cobb hotspot and the juan de fuca ridge axial seamount located along the central portion of the juan de fuca ridge axis and at the eastern end of the cobb eickelberg seamount chain is the current center of the cobb hotspot the axial seamount lavas are transitional between n type and e type mid ocean ridge basalt morb characteristics that they share with lavas along the rest of the juan de fuca ridge there are however subtle but distinct differences between the seamount lavas and those of the adjoining ridge segments these include higher na2o cao and sr at a given mgo content and lower silica saturation in the seamount lavas as compared with the ridge lavas lava chemistry and bathymetry indicate that axial seamount is a discrete volcanic unit with a more productive shallow magmatic plumbing system separate from the adjacent ridge segments these high magma supply rates have sustained a continuously replenished steady state magma reservoir that has erupted remarkably homogeneous lavas over a long time period despite this classic association of spreading center and hotspot volcanic activity there is no evidence in the lavas for geochemical or isotopic enrichment typical of hotspot or mantle plume activity the differences in composition between the axial seamount lavas and the juan de fuca ridge lavas are attributed to melting processes rather than to any fundamental differences in their mantle source compositions the higher magma production rates higher sr and lower silica saturation in the seamount lavas relative to the ridge lavas are thought to be a consequence of melt initiation at greater depths the melting column producing the seamount lavas is thought to be initiated in the stability field of spinel peridotite whereas the ridge lavas are produced from a melting column initiated at shallower levels possibly within or close to the stability field of plagioclase peridotite implicit in this interpretation is the conclusion that the juan de fuca ridge lavas and by analogy most morb are generated at shallow mantle levels mostly within the stability field of plagioclase peridotite this interpretation also requires that for the upwelling mantle to intersect the solidus at different depths the mantle supplying axial seamount must be hotter than the rest of the juan de fuca ridge axial seamount therefore reflects a thermal anomaly in the mantle rather than a geochemically enriched ocean island basalt type mantle plume

interaction of mantle plumes and migrating mid ocean ridges implications for the galapagos plume ridge system we investigate the three dimensional interaction of mantle plumes and migrating mid ocean ridges with variable viscosity numerical models numerical models predict that along axis plume width w and maximum distance of plume ridge interaction xmax scale with q u where q is plume source volume flux and u is ridge full spreading rate both w and xmax increase with buoyancy number b which reflects the strength of gravitational versus plate driven spreading scaling laws derived for stationary ridges in steady state with near ridge plumes are consistent with those obtained from independent studies of ribe 1996 in the case of a migrating ridge the distance of plume ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate given the mildly buoyant and relatively viscous plumes investigated here the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow from observed plume widths of the galapagos plume migrating ridge system our scaling laws yield estimates of galapagos plume volume flux of 5 16 106km3 m y 1 and a buoyancy flux of 2 103 kg s 1 model results suggest that the observed increase in bathymetric and mantle bouguer gravity anomalies along cocos plate isochrons with increasing isochron age is due to higher crustal production when the galapagos ridge axis was closer to the plume several million years ago the anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly 50 100 c and moderate radius 100 200 km predictions of the along axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but instead may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source

the rise and fall of axial highs during ridge jumps 1 we simulate jumps of ocean spreading centers with axial high topography using elastoplastic thin plate flexure models processes considered include ridge abandonment the breaking of a stressed plate on the ridge flank and renewed spreading at the site of this break we compare model results to topography at the east pacific rise between 15 25 n and 16 n where there is strong evidence of a recent ridge jump at an apparently abandoned ridge gravity data do not suggest buoyant support of topography model deflections during cooling and melt solidification stages of ridge abandonment are of small vertical amplitude because of plate strengthening resulting in the preservation of a frozen fossil high the present day high is bounded by slopes with up to a 40 grade a scenario very difficult to achieve flexurally given generally accepted constraints on lithospheric strength we model these slopes by assuming that the height at which magma is accreted increases rapidly after the ridge jumps this increase is attributed to high overburden pressure on melt that resided in an initially deep magma chamber followed by a rapid increase in temperature and melt supply to the region shortly after spreading began the high is widest at the segment center suggesting that magmatic activity began near the center of the segment propagated south and then north the mantle bouguer anomaly exhibits a bull s eye pattern centered at the widest part of the high but the depth of the axis is nearly constant along the length of the segment we reconcile these observations by assigning different cross axis widths to a low density zone within the crust

plume capture by divergent plate motions implications for the distribution of hotspots geochemistry of mid ocean ridge basalts and estimates of the heat flux at the core mantle boundary abstract the coexistence of stationary mantle plumes with plate scale flow is problematic in geodynamics we present results from laboratory experiments aimed at understanding the effects of an imposed large scale circulation on thermal convection at high rayleigh number 106 ra 109 in a fluid with a temperature dependent viscosity in a large tank a layer of corn syrup is heated from below while being stirred by large scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank three regimes are observed depending on the ratio v of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary and on the ratio of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary when v 1 and 1 large scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes when v 10 and 100 plume formation is suppressed entirely and the heat flux is carried by a sheet like upwelling located in the center of the tank at intermediate v and depending on established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling heat transfer across the layer occurs through a combination of ascending plumes and large scale flow scaling analyses show that the bottom boundary layer thickness and in turn the basal heat flux q depend on the peclet number pe and when 10 q pe1 2 and when 1 q pe 1 3 consistent with classical scalings when applied to the earth our results suggest that plate driven mantle flow focuses ascending plumes towards upwellings in the central pacific and africa as well as into mid ocean ridges furthermore plumes may be captured by strong upwelling flow beneath fast spreading ridges this behavior may explain why hotspots are more abundant near slow spreading ridges than fast spreading ridges and may also explain some observed variations of mid ocean ridge basalt morb geochemistry with spreading rate moreover our results suggest that a potentially significant fraction of the core heat flux is due to plumes that are drawn into upwelling flows beneath ridges and not observed as hotspots

crustal structure of the tuamotu plateau 15 s and implications for its origin we investigate the sedimentary and volcanic structure of the tuamotu plateau with multichannel seismic seismic refraction and gravity data along a ship track crossing the plateau near 15 s the volcanic basement of the central portion of the plateau is capped with a 1 to 2 km thick sediment layer composed of two compositional sequences the uppermost sequence with semblance derived p wave velocities of 1 6 1 9 km s and thicknesses of 0 2 0 9 km is composed of pelagic sediments the underlying sequence with velocities 2 5 3 5 km s and thicknesses of 0 5 1 5 km is composed of limestone and volcaniclastic sediments sonobuoy refraction data show the upper 1 km of the volcanic basement to have velocities 4 5 5 5 km s the gravity data indicate that the platform is compensated by an elastic lithosphere with effective thickness 5 5 km and that the volcanic thickness is 9 10 km thicker than normal oceanic crust with a volume of 2 0 2 6 l06 km3 the inferred eruption rates of 0 1 0 13 km3 yr are comparable to those of the hawaiian and marquesas island chains but substantially less than those of many oceanic plateaus radiometric and paleontological ages for the plateau and geomagnetic dates of the surrounding seafloor indicate that the northwestern portion of the plateau formed 600 km off the axis of the paleo pacific farallon spreading center on lithosphere of age 10 20 ma linear volcanic ridges and scarps bounding deep sediment filled basins however are similar to features of oceanic plateaus which formed at or near accretionary plate boundaries we suggest that these volcanic ridges and the gross plateau like morphology were formed by magma that was channelled along the lithospheric discontinuities left behind by a southward propagating rift segment of the nearby spreading center we attribute the formation of the northwestern portion of the tuamotu plateau to the passage of two hotspots during times 50 30 ma as they migrated beneath the pacific plate but remained west of the tuamotu propagator

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

nd and pb isotopic variations through the upper oceanic crust in dsdp odp hole 504b costa rica rift abstract the dsdp odp hole 504b drilled in the 5 9 ma southern flank of the costa rica rift represents the deepest section through modern ocean floor basaltic basement the hole penetrates a 570 m thick volcanic zone a 210 m thick transition zone of volcanic rocks and dykes and 1056 m of dykes a representative selection of these basalt types has been investigated with respect to nd and pb isotopes the nd of the basalts varies from 7 62 to 11 16 this range in the nd isotope composition represents about 67 of the total range reported for pacific morb the pb isotope composition also shows significant variation with 206 pb 204 pb varying from 17 90 to 18 82 the isotopic data show that a small volume of enriched mantle existed in the source the large ranges in isotopic composition in a single drill hole demonstrate the importance of small scale mantle heterogeneities in the petrogenesis of morb fractional melting and extraction of small magma batches by channelled flow and small short lived crustal magma reservoirs with limited potential for mixing of the mantle derived magmas are favored by these isotopic data

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

fossil hot spot ridge interaction in the musicians seamount province geophysical investigations of hot spot volcanism at volcanic elongated ridges the musicians seamount province is a group of volcanic elongated ridges vers and single seamounts located north of the hawaiian chain a 327 trending seamount chain defines the western part of the province and has been interpreted as the expression of a cretaceous hot spot beneath the northward moving pacific plate to the east elongated e w striking ridges dominate the morphology in 1999 wide angle seismic data were collected across two 400 km long vers we present tomographic images of the volcanic edifices which indicate that crustal thickening occurs in oceanic layer 2 rather than in layer 3 this extrusive style of volcanism appears to strongly contrast with the formation processes of aseismic ridges where crustal thickening is mostly accommodated by intrusive underplating high resolution bathymetry was also collected which yields a detailed image of the morphology of the vers from the occurrence of flat top guyots and from the unique geomorphologic setting two independent age constraints for the pacific crust during the cretaceous quiet zone are obtained allowing a tectonic reconstruction for the formation of the musicians vers hot spot ridge interaction leads to asthenosphere channeling from the plume to the nearby spreading center over a maximum distance of 400 km the musicians vers were formed by mainly extrusive volcanism on top of this melt generating channel the proposed formation model may be applicable to a number of observed volcanic ridges in the pacific including the tuamotu isles the eastern portion of the foundation chain and the western termination of the salas y gomez seamount chain

the dynamics of off axis plume ridge interaction in the uppermost mantle two decades of geochemical and geophysical observations have led to the plume channel model whereby buoyant off axis mantle plumes feed and interact with diverging plates at mid ocean ridge axes which act as sinks of upper mantle material here we present results from two dimensional 2 d numerical experiments which incorporate the essential physics and fluid dynamic aspects of the plume ridge upper mantle system in order to test the feasibility of this plume ridge interaction model specifically experiments test the relative importance of physical effects such as strong viscosity variations and thermal and chemical buoyancy forcing in plume ridge dynamics results indicate that both transient and steady state connections may be established between off axis plumes and ridges for a range of realistic mantle conditions the presence of a strongly sloping rheological boundary layer rbl is a necessary condition for long term communication between an off axis buoyant upwelling and a spreading ridge the flux of buoyant material to the ridge is also shown to increase with increasing plume to mantle density contrast decreasing plume viscosity and smaller plume ridge separation distances plume ridge interaction regimes are defined based on the competing effects of plate driven and buoyancy driven flow thermal erosion of the viscous lithosphere strongly inhibits long term plume ridge interaction by enhancing the plates ability to deflect the plume both head and conduit away from the ridge axis

melting dehydration and the geochemistry of off axis plume ridge interaction 1 we present results from a series of two dimensional numerical experiments in which synthetic melt compositions are calculated for a system in which a thermally buoyant off axis mantle plume interacts with a nearby ridge axis spatial gradients in synthetic melt properties are compared to observed spatial gradients in geochemistry from the easter salas y gomez system in an effort to constrain the dynamics of mantle flow in off axis plume ridge systems results indicate that observed gradients in radiogenic isotopic ratios between the ridge axis and the plume require significant heating of the ambient mantle adjacent to the plume this heating allows ambient mantle to melt off axis but also attenuates the flow from the plume to the ridge when increases in viscosity due to dehydration during melting are considered spatial gradients in the geochemical properties of synthetic melts become extremely sharp at odds with the observational data this may indicate that viscosity increases due to dehydration are not significant in off axis plume ridge systems

the geodynamic province of transitional lithosphere adjacent to magma poor continental margins two different types of transitional lithosphere have been documented along magma poor rifted margins one consists of apparently subcontinental mantle that has been exhumed brit tlely deformed and serpentinized during late stages of rifting a second is thinned 10 km con tinental crust which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth dependent extension in both cases it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter and thus that transitional lithosphere has relatively limited width here three representa tive cases of transitional lithosphere are examined one in the newfoundland iberia rift and one at goban spur both exhumed mantle and one off the angola congo margin thin continental crust flanked seaward by apparently exhumed lower continental crust exhumed mantle considering the geological and geophysical evidence it appears that depth dependent extension riftward flow of weak lower continental crust and or upper mantle may be a common phenomenon on magma poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed transitional lithosphere in this wide zone may consist of subcontinental mantle lower continental crust or some combination thereof depending on the strength profile of the pre rift continental lithosphere transitional lithosphere ceases to be emplaced i e final break up occurs only when emplacement of heat and melt from the rising asthenosphere becomes domi nant over lateral flow of the weak lower lithosphere this model implies a two stage break up first the rupture of the brittle continental crust and second the eventual separation of the ductile sub continental lithosphere which is coincident with emplacement of normal oceanic crust well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpenti nized exhumed mantle and such anomalies therefore are not diagnostic of oceanic crust where present the anomalies can be helpful in interpreting and dating the rifting history

anisotropic tomography around la reunion island from rayleigh waves in the western indian ocean the reunion hot spot is one of the most active volcanoes on earth temporal interactions between ridges and plumes have shaped the structure of the zone this study investigates the mantle structure using data from the reunion hotspot and upper mantle reunions unterer mantel rhum rum project which significantly increased the seismic coverage of the western part of the indian ocean for more than 1 year 57 ocean bottom seismometer stations and 23 temporary land stations were deployed over this area for each earthquake station path the rayleigh wave fundamental mode phase velocities were measured for the periods from 30 s to 300 s and group velocities for the period from 16 s to 250 s a three dimensional model of the shear velocity in the upper mantle was built in two steps first the path mean phase and group velocities were inverted to obtain regionalized velocity maps for each separate period then all of the phase and group velocity maps were combined and inverted at each grid point to obtain the local s wave velocity as a function of depth using a transdimensional inversion scheme the three dimensional anisotropic s wave velocity model has resolution down to 300 km in depth the tomographic model and surface tectonics are correlated down to 100 km in depth starting at 50 km in depth a slow velocity anomaly beneath rodrigues ridge and the east west orientation of the azimuthal anisotropy show a connection between reunion upwelling and the central indian ridge the slow velocity signature beneath la reunion is connected at greater depths 150 300 km with a large slow velocity zone beneath the entire mascarene basin this develops along a northeast direction following the general motion direction of the african plate these observations indicate nonisotropic spreading of hot plume material and dominant horizontal flow in the upper mantle beneath this area

87sr86sr and ree variations along the easter microplate boundaries south pacific application of multivariate statistical analyses to ridge segmentation abstract we report 87 sr 86 sr ree sc v cr and co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the easter microplate emp the latitudinal 87 sr 86 sr variation along the east and west rifts closely parallels that of the pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the east pacific rise epr the data also further support the binary mantle mixing model previously proposed we evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses the data base includes major and trace elements ranging from compatible e g cr to highly incompatible e g la and the pb and sr isotope ratios multivariate classification of these geochemical variables in q mode sample space allows to clearly distinguish major element grouping from those of moderate to highly incompatible elements and the isotope ratios reflecting mantle source heterogeneities present in the region the power of this multivariate geochemical classification is best represented in two dimensional space by combining principal components analysis pca with cluster analysis ca illustrated in the form of a dendrogram the method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities using the variance covariance matrix and correlation matrix in r mode we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation both discriminant analysis and pca further indicate a close relationship between the geochemical anomaly located on the east rift of the emp and sala y gomez and easter island to a lesser extent the tuamotu chain whereas there is no close affinity between the emp and the more distant juan fernandez san felix or society islands in the isotopic space considered this provides further support that the geochemical anomaly located on the east rift of the emp may be related to the influence of a plume located either beneath the east rift or in the vicinity of sala y gomez or easter island in the last two models the plume would be laterally deflected at shallow depth towards the epr emp spreading complex an analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle the flow between the plume and the east rift of the emp must be more confined or channelled the possibility that the plume would be located directly beneath the east rift cannot be ruled out but appears less likely

en echelon volcanic elongate ridges connecting intraplate foundation chain volcanism to the pacific antarctic spreading center abstract the development of volcanic elongate ridges vers could be controlled in many cases by complex processes related to interaction between mantle plumes and spreading centers we report here measured ages for the foundation chain se pacific showing that volcanism along morphologically distinct vers can develop occasionally as rapidly formed continuous lines of coeval volcanism extending from a region of intraplate volcanism to the pacific antarctic spreading center but a significantly more dominant trend is for coeval yet structurally disconnected segments of foundation chain vers to develop in a series of en echelon ne sw elongate zones of coeval hotspot volcanism these elongate zones developed at intervals of approximately 1 myr while maintaining a basically steady state orientation and size as the pacific antarctic spreading center migrated continually closer to the foundation plume hotspot we infer from this new information that foundation chain ver development was controlled by an interplay between short lived local factors e g location of nearest spreading center segment lithospheric stress and long lived attributes of the foundation plume hotspot e g size orientation periodicity

mantle convection plates and hotspots abstract more than 20 years after their discovery plates and hotspots are still awaiting a consistent dynamic model both imply large viscosity variations in the mantle sustained efforts have been targeted on modeling on computer as well as in the laboratory convection in fluids with temperature and pressure dependent viscosity the main results of these studies are reviewed and summarized in a few simple rules several properties of plates such as ridge push or heat flow distribution are well accounted for by convection models with temperature dependent viscosity however the process of subduction is not properly explained by these models one of the simple rules deduced from the study of convection and temperature dependent viscosity states that the viscosity drop across the lower boundary layer adjusts to a low value less than 10 this constrasts strongly with the viscosity variations required by current hotspot formation models which are typically 100 times larger it is suggested that this paradox can be solved if subduction is properly included in the convection models

madagascar basalts tracking oceanic and continental sources extensive upper cretaceous volcanism in southern madagascar was fed in part by mantle sources resembling those expressed today in the indian ocean at marion and prince edward islands and on the central southwest indian ridge in addition very low end t to 17 4 high 87sr 86sr t to 0 72126 tholeiites in southwestern madagascar were variably but highly contaminated by ancient continental material broadly like that affecting the bushe and poladpur formations of the later deccan traps in india alkalic dikes in southwestern madagascar have a rough analogue in the mahabaleshwar formation of the deccan in that they document the influence of a low 206pb 204pb negative end relatively low 87sr 86sr reservoir a very similar reservoir is manifested at present in mid ocean ridge basalts on the central southwest indian ridge near 40 e the original location of this end member appears likely to have been in the madagascan lithospheric mantle a portion of which may have been removed in the middle cretaceous by the action of the marion hotspot or the rifting of indo madagascar an origin within the hotspot itself also may be possible however recent products of the hotspot appear to lack completely the necessary low 206pb 204pb low end signatures

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

martian tharsis bulge insight to mantle related processes abstract when examining the tectonics of the one plate planet mars it is possible to test and even to develop further some ideas with respect to earth s plate tectonic paradigm the definition of terrestrial plate movements does not unambiguously identify the actual mantle processes involved one way to leam more about the endogenous development of planets is to undertake comparative studies the main tectonic structures of mars are interpreted here to give information concerning mantle processes which may relate to those processes driving earth s plate tectonics but which in the case of mars have been insufficient to break martian lithosphere into plates but which have been sufficient to cause rift block tectonics and to build the tharsis bulge

interaction between the mid atlantic ridge and the azores hot spot during the last 85 myr emplacement and rifting of the hot spot derived plateaus 1 multiple and single beam bathymetric data are compiled over the azores plateau to produce a 1 km 1 km grid between latitudes 32 n and 49 n and longitudes 22 w and 43 w mantle bouguer anomalies are then calculated from this grid and the satellite derived gravity these grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis the elevated seafloor of the azores plateau is interpreted as resulting from the interaction of a mantle plume with the mid atlantic ridge mar the presence of a large region of elevated seafloor associated with a thick crust between the great meteor seamounts and the azores platform on the africa plate and less developed conjugate structures on the north america plate favors genetic relations between these hot spot derived structures this suggests that a ridge hot spot interaction has occurred in this region since 85 ma this interaction migrated northward along the ridge axis as a result of the sse absolute motion of the africa plate following a direction grossly parallel to the orientation of the mar kinematic reconstructions from chron 13 35 ma to the present allow a proposal that the formation of the azores plateau began around 20 ma and ended around 7 ma a sharp bathymetric step is associated with the beginning of important melt supply around 20 ma the excess of melt production is controlled by the interaction of the ridge and hot spot melting zones the geometry and distribution of the smaller scale features on the plateau record episodic variations of the hot spot melt production the periodicity of these variations is about 3 5 myr following the rapid decrease of widespread volcanism the plateau was subsequently rifted from north to south by the mid atlantic ridge since 7 ma this rifting begins when the mar melting zone is progressively shifted away from the 200 km plume thermal anomaly these results bear important consequences on the motion of the africa plate relative to the azores hot spot they also provide an explanation to the asymmetric geochemical signature of the azores hot spot along the mid atlantic ridge

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

galapagos hot spot spreading center system 2 87sr 86 sr and large ion lithophile element variations 85 w 101 w thirty eight samples of basalts dredged from the galapagos spreading center gsc between 85 w and 101 5 w longitudes have been analyzed for k rb cs ba and sr contents as well as for 87sr 86sr ratios basalts between 85 w and 89 w segment of the gsc range from 0 7025 to 0 7028 in 87sr 86sr and those between 95 5 w and 101 5 w range from 0 7026 to 0 7027 these basalts are characterized by lree depleted patterns basalts from 89 w to 95 5 w segment having slightly lree depleted to lree enriched patterns la sm e f between 0 8 and 2 4 range from 0 7027 to 0 7031 in 87sr 86sr the longitudinal 87sr 86sr variation shows a maximum at about 92 w where the gsc is nearest to darwin island the alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles but these occur at about 91 w the apparent shift of 87sr 86sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting the highest 87sr 86sr from the gsc is only slightly lower than the highest 87sr 86sr for tholeiitic basalts from the galapagos archipelago several possible hypotheses have been examined to account for these geochemical and sr isotopic variations in the light of the presently available data the most promising explanation seems to be a binary mixing of hot spot or plume type material with the source of normal mid ocean ridge basalts and resulting rift propagation both end members seem to be heterogeneous and therefore it is at present difficult to put severe constraints on this binary mixing nevertheless the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91 w and 92 5 w fracture zones toward the rift

metamorphosed pb zn ag ores of the keketale vms deposit nw china evidence from ore textures fluid inclusions geochronology and pyrite compositions abstract the keketale pb zn deposit is located in the devonian volcanic sedimentary maizi basin of the altay orogenic belt the mineralization at keketale is hosted in marbles and deformed volcanic tuffs and biotite garnet chlorite schists folded into a series of overturned synclines formed in multiple deformation events keketale contains economic amounts of pb 0 89 mt 1 51 wt zn 1 94 mt 3 16 wt and ag 650 t 40 g t detailed petrographic studies have defined two main generations of sulfide development the banded pyrite of the early stage a is commonly stratiform with minor galena sphalerite and chalcopyrite stage b is characterized by a large amount of polymetallic sulfides including pyrrhotite chalcopyrite sphalerite and galena with minor pyrite hosted in quartz veins three types of fluid inclusions fis including mixed carbonic aqueous c type pure carbonic pc type and aqueous w type have been recognized in quartz of stage b the c type fis have homogenization temperatures of 150 326 c and salinities of 0 2 16 6 wt nacl equivalent the pc type fis are dominated by co2 with minor ch4 and n2 and have initial ice melting temperatures of 57 5 to 56 7 c co2 homogenization temperatures of 11 14 1 c the w type primary fis were completely homogenized at temperatures of 124 359 c with salinities of 5 0 14 6 wt nacl equivalent such co2 rich fluid inclusions are consistent with those discovered in orogenic type deposits in the altay area and elsewhere muscovite separates from the polymetallic quartz veinlets of stage b yield a well defined 40ar 39ar isotopic plateau age of 259 33 2 56 ma with an isochron age of 259 62 2 65 ma this age is coeval with the closure of the paleo asia ocean and reactivation of the ertix fault system la icp ms analyses of two generations of pyrite indicate that the banded pyrite of stage a is relatively depleted in metallic elements and contains low contents of cu 0 39 ppm ag 0 20 ppm au below the detection limits pb 17 43 ppm and zn 14 38 ppm whereas the pyrite in quartz polymetallic sulfide veinlets of the stage b is relatively rich in metallic elements e g cu 2 56 ppm ag 3 07 ppm au 0 01 ppm pb 1047 ppm and zn 1136 ppm the trace amounts of cu pb zn au and ag are interpreted to have been initially locked in the lattice of type a pyrite and then liberated and precipitated as micromineral inclusions with type b pyrite during subsequent metamorphism and deformation two key factors are considered vital to the formation of economic ores of the keketale pb zn deposit namely the original devonian banded pyrite formed in a vms system and subsequent permian deformation and metamorphic processes that liberated cu pb zn au and ag from the lattice of type a pyrite to form galena sphalerite and chalcopyrite with minor muscovite in quartz veinlets the model provides a new interpretation of vms pb zn deposit occurring in back arc basin environments followed by collision and new insights into the unique regional fe cu pb zn au mineralization in the altay orogenic belt

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

helium isotope ratios in easter microplate basalts abstract 3 he 4 he ratios in easter microplate basalt glasses show clear evidence of the effects of a mantle plume the east rift of the microplate between 26 and 28 s identified by la sm sr and pb isotopes and ridge crest elevation as the region of maximum plume influence has 3 he 4 he ratios spanning the entire range from 7 5 to 11 7 r a the easter microplate is the only section of the entire east pacific rise that is associated with a known hotspot track mantle plume and has elevated 3 he 4 he ratios although most of the west rift basalts contain morb helium 8 0 8 7 r a the basalt closest to the east rift has an elevated 3 he 4 he ratio 11 3 r a consistent with a significant plume component the diversity in isotopic signatures also indicates that homogenization of isotopic anomalies does not occur even in this region of super fast spreading the overall 3 he 4 he 206 pb 204 pb and 3 he 4 he 87 sr 86 sr trends have positive correlations although the high 3 he 4 he 87 sr 86 sr and 206 pb 204 pb component is not a single value but rather a broad band the apparent decoupling between the he and sr isotope distribution is modeled in the context of a plume source migrating ridge sink during channeling of the plume toward the ridge helium is preferentially lost from the center of the channeled plume resulting in lower he pb and he sr concentration ratios in the high 3 he 4 he component mixing trajectories in he sr isotopic space between a lile depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may explain the isotope systematics of plume ridge interactions in the context of modern theories of plume dynamics

three dimensional geometry of axial magma chamber roof and faults at lucky strike volcano on the mid atlantic ridge we present results from three dimensional 3 d processing of seismic reflection data acquired in june 2005 over the lucky strike volcano on the mid atlantic ridge as a part of the seismic study for monitoring of the mid atlantic ridge survey we use a 3 d tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time migrated seismic volume and provide 3 d geometry of the axial magma chamber roof fault reflectors and layer 2a gradient marker we also generate a high resolution bathymetric map using the seismic reflection data the magma chamber roof is imaged at 3 4 0 4 km depth beneath the volcano and major faults are imaged with dips ranging between 33 and 50 the magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10 of the total plate separation shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces

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

cumulate gabbros from the southwest indian ridge 54 s 7 16 e implications for magmatic processes at a slow spreading ridge a diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the southwest indian ridge 54 s 7 16 e between the islas orcadas and shaka fracture zones the cumulus nature of the gabbroic rocks in the suite is indicated by phase modal and cryptic layering igneous lamination and low incompatible element abundances we present a mass balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk rock major element compositions the model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid calculated proportions of trapped liquid range from 3 15 values that are characteristic of adcumulates to mesocumulates models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids thus explaining the high tio2 contents observed in magnesian clinopyroxenes cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers but disequilibrium plagioclase compositions suggest some crystallization at greater depth furthermore basalt compositions projected onto the olivine clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures from less than 10 to greater than 20 kb as well as polybaric fractional crystallization we suggest that the southwest indian ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros the adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large but ephemeral magma chambers below segments of the southwest indian ridge

neodymium isotopic evidence for galapagos hotspot spreading centre system evolution nd and sr isotope variations along the galapagos spreading centre support morgan s preferential subcrustal flow channel connecting the galapagos mantle plume with the northeastward migrating ridge axis decoupling of la sm and nd isotope variations near 91 w further suggests that basalts are derived from smaller degrees of melting and are deeper beneath this elevated part of the ridge

the effects of shallow rheological boundaries in the upper mantle on inducing shorter time scales of diapiric flows the authors report on a study of pulsating diapiric flow in the upper mantle they study the effect of introducing new phase boundaries in the mantle on such flow basic models have an interface at a depth of 670 km in the rheological properties the authors study the effect of introducing another phase boundary at a depth close to 400 km

upper mantle heterogeneities and dynamics the width of geochemical and residual elevation anomalies along the mid ocean ridge system decrease as both the distance between migrating ridge segments and related hotspots and the local geochemical dispersion in the anomalies increase these observations provide further evidence that flows of mantle plumes in the upper mantle are deflected by migrating ridge sinks once they override hotspots

mantle mixing and continental breakup magmatism abstract the frequent formation of large igneous provinces during the opening of the atlantic ocean is a surface manifestation of the thermal and chemical state of convecting mantle beneath the supercontinent pangea recent geochemical and geophysical findings from the north atlantic igneous province all point to the significant role of incomplete mantle mixing in igneous petrogenesis on the basis of a whole mantle convection model with chemical tracers i demonstrate that sublithospheric convection driven by surface cooling can bring up dense fertile mantle without a thermal anomaly when small scale convection in the upper mantle breaks down into the lower mantle strong counter upwelling takes place entraining a large amount of dense crustal fragments accumulated at the base of the mantle transition zone this multi scale mantle mixing could potentially explain a variety of hotspot phenomenology as well as the formation of both volcanic and non volcanic rifted margins with a spatially and temporally varying distribution of fertile mantle

analysis of geometry of volcanoes and faults in terceira island azores evidence for reactivation tectonics at the eur afr plate boundary in the azores triple junction abstract the late pliocene to quaternary 5 ma volcanism in the central and eastern azores archipelago is related to the eurasia africa divergence but a clear deformation pattern has not yet been established at this location this work focuses on the contribution of synthetic aperture radar sar scenes and digital elevation models dem complemented with geophysical geodetic and morpho structural data to establish the geometric relationships between volcanic edifices and tectonic structures in the central and eastern azores archipelago bathymetric data were also used to extend field observations to the significant submarine area of the azores plateau strikes of extension fractures directly observed or inferred from elongated volcanic vents or linear volcanic clusters in terceira island indicate that volcanism is mainly controlled by regional extension as given by nuvel 1a plate motion model additionally other directions were also detected for extension fractures around the santa barbara volcano defining a radial pattern and in the central part of the island exhibiting an s shape pattern although most of the volcanic vents are controlled by extension fractures some seem to be controlled by faulting such as the case of the ones rooted in releasing bends along strike slip or oblique slip faults in the central part of the island concerning the azores plateau most of the structures have directions that do not directly fit with present day direction of relative motion n70 between eurasia and africa directions ranging from n110 to n125 found mainly along the terceira rift are interpreted as ancient transform directions reactivating as transtensional fault zones due to the present day plate motion n s directions are also visible in the plateau being interpreted as former middle oceanic rift faults reactivated as left lateral fault zones these results contrast with the volcanic expression in other hotspot dominated oceanic islands such as the canaries or the hawaii islands chain probably due to the tectonic complexity promoted by the azores triple junction instability through time

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

evidence for age and evolution of corner seamounts and great meteor seamount chain from multibeam bathymetry the comer seamounts in the western north atlantic and great meteor seamount chain in the eastern north atlantic are thought to progress in age from late cretaceous through late cenozoic they both presumably formed by volcanism above the new england hotspot when first the north american plate and then the mid atlantic ridge axis and african plate moved over the hotspot high resolution multibeam bathymetry of the seamounts shows geomorphic features such as guyots terraces and a base level plateau cruiser plateau that we interpret to have formed at sea level we have backtracked these features to sea level along the north atlantic crustal age depth curve in order to estimate their ages the derived age pattern of volcanism indicates formation of the comer seamounts at ca 80 ma to 76 ma with migration of the mid atlantic ridge plate boundary over the hotspot and formation of the cruiser plateau about 76 ma seamount ages suggest that subsequent volcanism on the african plate moved first northward in the late cretaceous to early cenozoic plato tyro and atlantis seamount groups then southward to great meteor seamount in the late cenozoic recurrent volcanism appears to have occurred at some seamounts up to 20 30 m y after their initial passage over the hotspot it would thus appear that intralithospheric conduits can link the hotspot to old seamounts several hundred kilometers away

latitudinal electron precipitation patterns during large and small imf magnitudes for northward imf conditions we demonstrate that there are distinct differences in the electron precipitation patterns or the polar cap size geomagnetic activity and field aligned currents in the highest latitude region for small and large imf bz values when the imf bz component is positive first during periods of weakly northward imf there is a distinct area in the highest latitude region in which the electron precipitation is absent except for the polar rain by contrast during strongly northward imf the entire polar region is often filled with burst type soft electron precipitations second geomagnetic disturbances and field aligned current intensitites in the highest latitude region 80 in geomagnetic latitude are less during a weak imf bz condition than those during a strongly northward imf bz condition geomagnetic activity in the auroral zone for both conditions is absent or very weak

seismic shear wave structure of the uppermost mantle beneath the mohns ridge crust produced at mid ocean ridges with full spreading rates less than 20 mm yr is observed to be only 0 4 km thick well below the global average of 6 7 km for oceanic crust produced at faster spreading rates the origin of this difference is unknown but is speculated to result from a thicker thermal boundary layer at the axis of the slowest spreading ridges that either inhibits shallow melting or melt extraction we present an analysis of regional broadband data predominately love and rayleigh waves collected along the very slow spreading mohns ridge where crustal thickness is 4 km in the norwegian greenland sea the seismic data constrain lithospheric and asthenospheric velocities for lithospheric ages from 0 to 25 ma we find lithospheric thickness to closely match the prediction of a simple ridge half space thermal model via the temperature and pressure effects on the seismic properties of mantle materials asthenospheric shear wave velocities are consistent with this thermal model plus 2 melt at the youngest ages just at the top of the mantle a thin zone with velocities intermediate between those of mantle and gabbroic rocks suggests that some melt is frozen into the mantle as might occur if a thin axial lithospheric lid inhibits melt extraction from the mantle and the melt is subsequently frozen into the mantle if this process occurs to some degree at all slow to very slow spreading ridges then there may be spatially large reservoirs of unaccounted for gabbroic melt frozen into the mantle throughout many ocean basins

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

magmatic segmentation of the northern knipovich ridge evidence for high pressure fractionation at an ultraslow spreading ridge 1 the ultraslow spreading knipovich ridge shows axial topographic highs that are associated with off axis linear arrays of seamounts that parallel the spreading direction these linear arrays suggest that the segmentation has been stable for at least 7 8 m y axial topographic highs are marked with distinct gravity anomalies suggesting that they represent thicker crust and hence volcanic segment centers dredging on the northern knipovich ridge between 76 30 and 77 50 n shows that the axial and off axial seamounts and the deeper axial rift valley between the segment centers are composed of volcanic rocks the basalts are subdivided into high intermediate and low feo groups low feo basalts with generally more enriched radiogenic isotope ratios and ree patterns were only dredged midway between segment centers whereas dredges close to the segment centers comprised high and intermediate feo basalts the variations in isotopes and ree patterns suggest that the mantle below the northern knipovich ridge is heterogeneous and the along axis variations can be explained by higher degrees of melting of this heterogeneous source close to segment centers the high and intermediate feo basalts define different mgo feo liquid lines of descents that may reflect differences in the crystallization pressures or the water contents of the magmas similar water contents of the high and intermediate feo basalts suggest that the differences in feo primarily are a pressure effect and that the intermediate feo group fractionated at higher pressure than the high feo group the high feo basalts show both low and high pressure phenocryst assemblages suggesting crystallization at high pressures 6 7 kbar followed by shallow low pressure fractionation on the basis of the new data we propose that magmas at this ultraslow spreading ridge fractionate at mantle depth except at segment centers where magma productivity occasionally is high enough to sustain shallow level magma reservoirs

anomalous seafloor spreading of the southeast indian ridge near the amsterdam st paul plateau the amsterdam st paul plateau is bisected by the intermediate rate spreading southeast indian ridge and numerous geophysical and tectonic anomalies arise from the interactions of the amsterdam st paul hotspot and the spreading center the plate boundary geometry on the hotspot platform evolves rapidly on timescales 1 myr off axis volcanism is abundant the seafloor does not deepen away from the axis and transform faults do not have fracture zone extensions away from the hotspot platform the ridge transform geometry is typical of mid ocean ridges globally in contrast the amsterdam st paul plateau spreading segments are shorter they often overlap each other significantly and the intervening discontinuities are smaller more ephemeral and more migratory abyssal hills are smaller and less uniform on the hotspot platform than on neighboring spreading segments from gravity and isostasy analysis the average thickness of the platform crust is 10 km approximately 50 thicker than that of typical oceanic crust most of the isostatic compensation of the hotspot plateau occurs at the moho or within the lower crust and the effective elastic thickness of the plateau lithosphere is 1 6 km less than half that of adjacent spreading segments away from the platform some transform faults contain intratransform spreading centers on the platform the two transform faults have valleys which may be depocenters for abundant axial or off axis volcanism and mass wasting although not wellconstrained by magnetics coverage the amsterdam st paul hotspot appears to have been captured by the southeast indian ridge enhancing crustal production at the ridge since about 3 5 ma prior to this time the hotspot formed a line of smaller isolated volcanoes on older australian plate the underlying cause for the present day crustal accretion anomalies is the effect of melt generation from separate sources of mantle upwelling due to plate spreading and the hotspot which has a consequent effect of weakening the lithosphere

age progressive volcanism in the new england seamounts and the opening of the central atlantic ocean radiometric ages k ar and oar 39ar methods have been determined on dredged volcanic rocks from seven of the new england seamounts a prominent northwest southeast trend ing volcanic lineament in the northwestern atlantic ocean the oar 39ar total fusion and increnental heating ages show an increase in seamount construction age from southeast to northwest that is consistent with northwestward motion of the north american plate over a new england hot spot between 103 and 82 ma a linear volcano migration rate of 4 7 cm yr fits the seamount age distribution these ages fall within a longer age progression from the corner seamounts 70 to 75 ma at the eastern end of the new england seamounts to the youngest phase of volcanism in the white mountain igneous province new england 100 to 124 ma the new england hot spot estimated from the new radio metric ages and motion of north america in the hot spot reference frame to be near 28on 33ow probably generated a short line of mid to late tertiary age seamounts on the african plate but appears to be presently inactive the hot spot reference frame is used to calculate the motion of the north american plate away from the african plate from early cretaceous time to the present rominent magnetic anomalies recorded in the central atlantic seafloor give the positions of the spreading ridge for a bange of known ages hot spots that now underlie the african plate in the eastern central atlantic new england canary madeira cape verde azores could have produced cretaceous seamount and island chains on the north american plate during the early opening of the central atlantic each of these hot spots has been overridden by spreading ridges at pre dictable times some of these hot spot crossings are expressed as geochemical anomalies in the oceanic crust now far removed from spreading ridges

surface wave tomography of the upper mantle beneath the reykjanes ridge with implications for ridge hot spot interaction 1 we modeled fundamental mode love and rayleigh waves to study the seismic properties of the upper mantle beneath the reykjanes ridge these waves were generated by regional earthquakes occurring in the north atlantic to the south of iceland and were recorded by stations located on iceland over 12 000 measurements of the phase group arrival time and amplitude of narrow pass filtered waveforms over the period range of 9 5 100 s were used to solve for mantle shear wave velocity structure and anisotropy in a vertical plane oriented normal to the ridge axis the velocity structure contains a broad and deep low velocity zone in the upper mantle beneath the ridge a joint analysis of the seismic structure with gravity data reveals that the low velocities are consistent with elevated temperatures 75 our study shows that plume material spreads broadly outward beneath the reykjanes ridge from iceland and is not confined to a narrow lithospheric channel at distances 200 km from the ridge shear wave anisotropy indicates a predominant horizontal alignment of the fast axes of anisotropic crystals mainly above 50 km depth which can be interpreted as past horizontal ridge perpendicular flow within 200 km of the ridge the anisotropy indicates a general vertical alignment of the fast axes or an alignment such that the fast axes point along the ridge the transition to this type of anisotropy coincides with the appearance of increased hot spot ridge interaction 20 myr ago indicating that plume flow has largely disrupted mantle flow beneath the ridge since that time

evolution of the accretion processes along the mid atlantic ridge north of the azores since 5 5 ma an insight into the interactions between the ridge and the plume 1 high resolution bathymetry and gravity data north of the azores plateau show that this part of the north mid atlantic ridge is presently undergoing a phase of weak crustal production and magmatism most of the ridge segments are small and short lived suggesting a disrupted and highly variable accretion regime since anomaly 3a the influence of the nearby plume appears to be relatively minor and corresponds more to a weak thermal signal than to any major input of plume material and increased crustal production at the axis a period of increased magmatism was identified at the southern limit of the study area near 40 n around anomaly 5 this magmatic pulse caused the emplacement of a topographic high probably underlain by a thickened crust this pulse probably marks the northernmost and last significant arrival of material from the azores plume to the mar axis

the dynamics of plume ridge interaction iii the effects of ridge migration summary to investigate how the migration of ridges influences their interaction with nearby mantle plumes we study a thin layer model in which a point source analogous to a plume stem releases buoyant fluid at a rate q into a viscous corner flow driven by a ridge like surface boundary condition the ridge has a half spreading rate u and migrates relative to the point source at a constant speed um and azimuth from the plate spreading direction numerical solutions of the governing differential equation yield predictions of the lateral extent w of plume fluid along the ridge waist width which we compare with observed waist widths for 11 on and near ridge hotspots the main dynamical effect of ridge migration is to increase the maximum interaction distance for a ridge receding from a plume and to decrease this distance for a ridge approaching a plume models including normal age thickening and plume induced thinning of the lithosphere show that both effects have only a small maximum 20 per cent influence on the dynamics our results suggest that the volume fluxes q of five plumes far from ridges sala y gomez st helena great meteor reunion and kerguelen may be two to three times higher than previously estimated