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

chaotic subduction like downflows in a spherical model of convection in the earth s mantle numerical models of three dimensional nonlinear thermal convection of very viscous fluids have recently been developed in rectangular1 4 and spherical5 12 geometries as analogues of convection in the earth s mantle here we describe model calculations for a compressible fluid in a three dimensional spherical shell with 80 of the surface heat flow generated within the model mantle in agreement with estimates for the earth s mantle13 the rayleigh number and the numerical resolution for these calculations are greater than those of our previous studies9 10 of internally heated convection in a spherical shell our numerical solutions are strongly chaotic with surface planforms dominated by long curvilinear downflows reminiscent of the descending slabs in the earth s mantle although analogy to the earth s mantle is necessarily imperfect owing to for example the absence of variable viscosity and rheology and hence of lithospheric plates our results suggest that descending slabs play an important part in driving mantle convection and that their chaotic evolution may influence the spatial and temporal behaviour of plates and thus the dispersal and aggregation of continents

the proximity of hotspots to convergent and divergent plate boundaries an analysis of four different hotspot distributions ranging from morgan s 1972 original list of 19 to vogt s 1981 list of 117 reveals that the hotspots are preferentially located near divergent plate boundaries the probability of this proximity occurring by chance alone is quite remote less than 0 01 for all four hotspot distributions the same analysis also reveals that the hotspots are preferentially excluded from regions near convergent plate boundaries the probability of this exclusion occurring by chance alone is 0 1 or less for three out of the four distributions examined we interpret this behavior as being a consequence of the effects of large scale convective circulation on ascending mantle plumes mantle thermal plumes the most probable source of hotspots arise from instabilities in a basal thermal boundary layer plumes are suppressed from regions beneath convergent boundaries by descending flow and are entrained into the upwelling flow beneath spreading centers plate scale convective circulation driven by subduction may also advect mantle thermal plumes toward spreading centers

interaction of rising and sinking mantle plumes the frequency of plume formation in fully developed thermal convection is determined experimentally because the fluid has a temperature dependent viscosity the cold and hot thermal boundary layers have different thicknesses and viscosities as a result plumes are released from these layers with different frequencies there also appears to be a direct temporal interaction between the cold and hot boundary layers in particular whereas there is a single frequency for cold plume formation hot plumes form with multiple frequencies one of which is similar to the frequency of cold plumes the measurements thus suggest that cold sinking plumes may induce hot rising plumes

unusual features caused by lightning impact in west greenland two lightning impacts are described from an area near the inland ice in west greenland the first lightning blasted an outcrop of metacherts it subsequently split into two branches which traversed rock outcrops and boulders leaving behind two white almost straight lines 30 m and 14 m long respectively where all lichens and plants were burned away on the white lines the upper few millimetres of the traversed boulders were melted to a glass which subsequently peeled off by thermal expansion to leave a rough surface magnetic investigation of an amphibolite boulder found on the white line showed that a strong electric current indeed traversed the boulder a few years later a second lightning impacted on a mountaintop close to the first impact the second lightning left a trail on the rock surface covered by a thin layer of glass the glass displays spectacular colours ranging from metallic blue to red yellow and green

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

melting dehydration and the dynamics of off axis plume ridge interaction 1 we present results from a series of two dimensional numerical experiments in which a thermally buoyant off axis mantle plume interacts with a nearby ridge axis these experiments incorporate melting and a number of related dynamical feedbacks including energy loss to latent heating and fe depletion buoyancy as well as viscosity increases due to dehydration during melting results indicate that dehydration which acts to increase the viscosity of the residual mantle during melting profoundly impacts the morphology of flow between plume and ridge axis as upwelling plume material begins to melt it becomes significantly more viscous and readily accretes to the overlying lithosphere this creates a viscous plug that grows downward from the base of the lithosphere directly above the plume conduit plume material traveling to the ridge axis is then deflected horizontally at subsolidus depths by this plug allowing pristine unmelted plume material to reach the ridge axis experiments without dehydration show that the loss of thermal energy to latent heating during melting marginally decreases the flow of plume material along the base of the lithosphere to the ridge axis alternatively extraction of fe from the solid during melting slightly increases plume flow to the ridge these results also demonstrate that the flow of plume material to the ridge axis is sensitive to the slope of the base of the rheological lithosphere consistent with previous studies of off axis plume ridge interaction

multiscale seismic tomography and mantle dynamics abstract in this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs global and regional tomography shows that most of the slab materials under western pacific and east asia are stagnant in the mantle transition zone before finally collapsing down to the core mantle boundary as a result of large gravitational instability from phase transitions local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper mantle wedge indicating that geodynamic systems associated with arc magmatism and back arc spreading are related to deep processes such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab because most hotspots are located in poorly instrumented continental and oceanic regions 3 d crust and upper mantle structure is determined for only a few hotspots such as iceland yellowstone and eifel which are covered by seismic networks and plume like slow anomalies are revealed under those hotspots global tomography has revealed deep mantle plumes under the major hotspots such as hawaii iceland kerguelen south pacific and africa strong lateral heterogeneities are revealed at the bottom of the mantle which are associated with the deeply subducted slabs and the birth of mantle plumes a thorough understanding of the deep earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks particularly in the oceans

high rate injection is associated with the increase in u s mid continent seismicity an unprecedented increase in earthquakes in the u s mid continent began in 2009 many of these earthquakes have been documented as induced by wastewater injection we examine the relationship between wastewater injection and u s mid continent seismicity using a newly assembled injection well database for the central and eastern united states we find that the entire increase in earthquake rate is associated with fluid injection wells high rate injection wells 300 000 barrels per month are much more likely to be associated with earthquakes than lower rate wells at the scale of our study a well s cumulative injected volume monthly wellhead pressure depth and proximity to crystalline basement do not strongly correlate with earthquake association managing injection rates may be a useful tool to minimize the likelihood of induced earthquakes

sem observations on some sand fulgurites from northern australia abstract sand fulgurites fragments from a siliceous coastal dune field in north queensland have been examined with a scanning electron microscope sem the fulgurites show a number of morphological and textural features which can be related to the process of their formation when lightning strikes sandy sediments the outer surfaces of the fulgurite tubes possess a rough surface texture due to numerous vesicles and partly fused adhering quartz grains whereas the inner surface of the central lumen is typically much smoother and has a vitreous appearance zip like trails needles or threads of silica and clusters of iron stained quartz silt particles have been observed on the walls of the lumen but sem analysis has provided no evidence that the fulgurites examined were formed by fusion f quartz sand around plant roots

a comparison of seismicity rates and fluid injection operations in oklahoma and california implications for crustal stresses fluid injection into deep wellbores can increase pore pressure reduce effective stress and trigger earthquakes the extent of the seismogenic response to injection provides insight into how close faults are to failure in the injection affected area the seismogenic response to injection operations in hydrocarbon basins is examined in california and oklahoma changes in spatial and temporal seismicity rates are tested for significant variations and timing and location of such variations are determined based on nonparametric modeling of background seismicity rates oklahoma has experienced a recent surge of seismic events which exceeded the 95 confidence limit of poissonian background rates in c 2010 annual injection volumes in oklahoma increased systematically between 1998 and 2013 and have been connected to several earthquake sequences in california injection volumes increased monotonically between 2001 and 2009 however the seismogenic response was limited and was devoid of large scale background rate increase a detailed comparison of injection parameters in oklahoma and california included well density wellhead pressures peak and cumulative rates and injection depths no detectable difference was found that could readily explain the observed changes in seismicity rate in oklahoma and the lack thereof in california a strongly different seismogenic response to similar pressure perturbations indicates that the injection parameters considered are only of secondary influence on the resulting earthquake activity the primary controls on injection induced earthquakes might be the specific geologic setting and the stress state on nearby faults

lightning as a geomorphic agent on mountain summits evidence from southern africa abstract the presence of angular bedrock derived debris on mountain summits worldwide has usually been associated with present or past periglacial frost shattering thermal fracturing and other climatically mediated weathering processes climatic inferences are commonly made based on such geomorphological evidence even if frost shattering and other processes are unlikely under present climatic conditions this paper questions this assumed genetic link between present past climate and production of angular bedrock derived debris by describing the geomorphological impacts of lightning strikes on exposed mountain summits using examples from the high drakensberg of eastern lesotho southern africa the impacts of lightning strikes are described which include the generation of angular fractured bedrock derived debris these impacts are identified in the field based on clear and unambiguous criteria that can be used to distinguish between lightning induced weathering processes and those processes associated with more typical alpine weathering this paper argues that lightning strikes are an important geomorphic agent of in particular low latitude mountain summits and that to make uncritical climatic inferences based on the presence of frost shattered debris on mountain summits is wholly erroneous

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

bifurcation of the yellowstone plume driven by subduction induced mantle flow in the northwestern united states two neighbouring volcanic chains the snake river plain and high lava plains decrease in age in opposing directions laboratory experiments show that diverging volcanic chains can form where vigorous mantle circulation beneath a subduction zone causes an upwelling mantle plume to split into two

hotspots and sunspots surface tracers of deep mantle convection in the earth and sun abstract the pattern of new appearances of hotspots on the earth is investigated using available age data the worldwide total of cenozoic and mesozoic hotspots predicted by extrapolation from the observed number of continental flood basalts is 40 in agreement with crough s counts there are found to be no true antipodal pairs in one interpretation of the data new appearances of hotspots occur first at high latitudes in both hemispheres and then migrate toward the equator shortly before the migration cycle is finished the next cycle begins a relative deficiency of hotspots however is observed in very low equatorial and very high polar regions in addition hotspots occupy two large hemispherical groups in longitude that slowly drift in the same direction as the axial body rotation an observed magnetic superchron seems to end when a new hotspot migration cycle in latitude begins but consecutive superchrons show opposite polarity although the hotspot data are rather sparse their suggested patterns resemble the well known patterns of complexes of active regions on the sun as exemplified by proxy data such as sunspots both the earth and the sun possess large convecting rotating and magnetized mantles in which the characteristic surface tracers are believed to reflect the patterns of convective phenomena very deep in the mantle even though the tracers themselves hotspots and sunspots are certainly not analogs of each other the present study supports existing theoretical ideas that the large scale patterns of deep mantle convection in the earth and sun may fundamentally resemble each other despite the enormous difference in their molecular viscosities magnetic polarity reversal histories also show close similarities in the two bodies suggesting the operation of basically similar convective dynamos however the sun has displayed no known analog of the earth s magnetically disturbed intervals

a century of induced earthquakes in oklahoma seismicity rates have increased sharply since 2009 in the central and eastern united states with especially high rates of activity in the state of oklahoma growing evidence indicates that many of these events are induced primarily by injection of wastewater in deep disposal wells the upsurge in activity has raised two questions what is the background rate of tectonic earthquakes in oklahoma how much has the rate varied throughout historical and early instrumental times in this article we show that 1 seismicity rates since 2009 surpass previously observed rates throughout the twentieth century 2 several lines of evidence suggest that most of the significant earthquakes in oklahoma during the twentieth century were likely induced by oil production activities as they exhibit statistically significant temporal and spatial correspondence with disposal wells and intensity measurements for the 1952 el reno earthquake and possibly the 1956 tulsa county earthquake follow the pattern observed in other induced earthquakes and 3 there is evidence for a low level of tectonic seismicity in southeastern oklahoma associated with the ouachita structural belt the 22 october 1882 choctaw nation earthquake for which we estimate m w 4 8 occurred in this zone online material a discussion of historical earthquakes and populations in oklahoma figures of intensity and earthquake location maps and a cumulative count of earthquakes in oklahoma and tables of accounts and felt reports

lightning induced shock lamellae in quartz using transmission electron microscopy we show that planar deformation lamellae occur within quartz in the substrate of a rock fulgurite i e a lightning derived glass these lamellae exist only in a narrow zone adjacent to the quartz fulgurite boundary and are comparable to planar deformation features shock lamellae caused by hypervelocity impacts of extra terrestrial objects our observations strongly suggest that the lamellae described here have been formed as a result of the fulgurite producing lightning strike this event must have generated a transient pressure pulse whose magnitude however is uncertain at this stage

antipodal hotspot pairs on the earth the results of statistical analyses performed on three published hotspot distributions suggest that significantly more hotspots occur as nearly antipodal pairs than is anticipated from a random distribution or from their association with geoid highs and divergent plate margins the observed number of antipodal hotspot pairs depends on the maximum allowable deviation from exact antipodality at a maximum deviation of 700 km 26 to 37 of hotspots form antipodal pairs in the published lists examined here significantly more than would be expected from the general hotspot distribution two possible mechanisms that might create such a distribution include 1 symmetry in the generation of mantle plumes and 2 melting related to antipodal focusing of seismic energy from large body impacts

bayesian identification of multiple seismic change points and varying seismic rates caused by induced seismicity the central and eastern united states ceus has experienced an abnormal increase in seismic activity which is believed to be related to anthropogenic activities the u s geological survey has acknowledged this situation and developed the ceus 2016 1 year seismic hazard model using the catalog of 2015 by assuming stationary seismicity in that period however due to the nonstationary nature of induced seismicity it is essential to identify change points for accurate probabilistic seismic hazard analysis psha we present a bayesian procedure to identify the most probable change points in seismicity and define their respective seismic rates it uses prior distributions in agreement with conventional psha and updates them with recent data to identify seismicity changes it can determine the change points in a regional scale and may incorporate different types of information in an objective manner it is first successfully tested with simulated data and then it is used to evaluate oklahoma s regional seismicity

phanerozoic hot spot traces and paleogeographic reconstructions of the siberian continent based on interaction with the african large low shear velocity province abstract we review intraplate magmatism in siberia and its folded surroundings from 480 ma to the present we describe several large igneous provinces lips and the intervals in which they were continuously formed within the limits of the siberian continent the altay sayan early paleozoic magmatic area 598 446 ma the altay sayan lip 408 393 ma the viluy lip 380 350 ma the barguzin vitim lip 310 275 ma the late paleozoic rift system of central asia 318 242 ma the siberian traps and west siberian rift system 250 249 ma the east mongolian and west trans baikalian lip 228 195 ma and a number of various aged late mesozoic and cenozoic rift zones and magmatic areas from 160 ma to the present day following lawver and muller 1994 kharin 2000 lawver et al 2002 and chernysheva et al 2005 we accept the position of the icelandic hot spot under the siberian trap area at the permo triassic boundary that enables us to estimate the geographic coordinates of the siberian trap location at 250 ma they are the same as iceland today presently the icelandic hot spot is situated above the african large low shear velocity province llsvp that indicates a hot mantle plume we suggest a set of paleogeographic reconstructions of the siberian continent for which we evaluate the paleolatitude based on the paleomagnetic data and estimate the paleolongitude position by placing siberia above the african llsvp furthermore we estimate the geographic coordinates for other ancient hot spots in the framework of the african llsvp that we consider to be responsible for the intraplate magmatism during different time periods in the phanerozoic eon available rare element and isotopic characteristics of the intraplate magmatic rocks of siberia enable us to determine three primary sources moderately depleted mantle prema enriched mantle em i and em ii of the mantle origin magma we propose that the model explains the interaction of the hot mantle plume including hot spots plume tails with the siberian intraplate magmatism areas throughout the phanerozoic eon

flattening of the sea floor depth age curve as a response to asthenospheric flow the flattening of sea floor depths from the age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1 previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than 80 myr ref 2 thermal rejuvenation of lithosphere that passes over stationary hotspots3 5 or a whole mantle flow forced by two converging plates6 we suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere applying the model to the pacific plate we show that a solution for flow in an asthenosphere low viscosity channel which is consumed by plate accretion and the subduction of lithosphere at trenches and replenished by near ridge upwelling and near ridge hotspots generates the observed 1 km of sea floor flattening for asthenospheric viscosities of 2 1018 pa s and an absolute pacific plate motion of 100 mm yr 1 our model can also explain the asymmetric subsidence of the south american and african plates away from the mid atlantic ridge

bayesian identification of soil strata in london clay identification of soil strata in the london clay formation lcf is of practical significance as it allows geotechnical property data within the same soil strata to be compared or used effectively the soil strata in lcf are generally determined based on their lithological characters and fossil contents with which geotechnical engineers are less familiar to assist geotechnical engineers in identifying the soil strata in lcf this paper aims to develop bayesian approaches to identification of soil strata in lcf using water content data equations are derived for the proposed bayesian approaches and the approaches are illustrated using a water content profile at st james s park london the proposed bayesian approaches are shown to correctly identify the soil strata in lcf in addition a sensitivity study is performed to explore the effect of data quantity i e both the measurement interval and number of measurements at the same depth in a water content profile and provide guidance on the water conten

distribution of mantle up welling determined from plate motions a case for large scale benard convection the number and geometric distribution of putative mantle up welling centers and the associated convection cell boundaries are determined from the lithospheric plate motions as given by the 14 euler poles of the observational nuvel model for an assumed distribution of up welling centers called cell cores the corresponding cell boundaries are constructed by a voronoi division of the spherical surface the resulting polygons are called benard cells by assuming the flow kinematics within a cell the viscous coupling between the flow and the plates is estimated and the euler poles for the plates are computed under the assumption of zero net torque the positions of the cell cores are optimized for the hs2 nuvel1 euler poles by a method of successive approximation subplex convergence to one of many local minima occurred typically after 20 000 iterations cell cores associated with the fourteen hs2 nuvel1 euler poles converge to a relatively small number of locations 8 to 10 depending on interpretation irrespective of the number of convection cells submitted for optimized distribution from 6 to 50 these locations are correlated with low seismic propagation velocities in tomography uniformly occur within hotspot provinces and may specifically be associated with the hawaiian iceland reunion kerguelen indian ocean easter island melanesia society islands south pacific azores cape verde canary islands tristan da cunha south atlantic balleny islands and possibly yellowstone hotspots it is shown that arbitrary euler poles cannot occur in association with mantle benard convection irrespective of the number and the distribution of convection cells nevertheless eight of the observational euler poles including the five that are accurately determined in hs2 nuvel1 australia cocos juan de fuca pacific and philippine are benard valid i e can be explained by our benard model five of the remaining six observational poles must be relocated within their error ellipses to become benard valid the eurasia pole alone appears to be in error by 115 and may actually lie near 40 n 154 e the collective results strongly suggest benard like mantle convection cells and that basal shear tractions are the primary factor in determining the directions of the plate motions as given by the euler poles the magnitudes of the computed euler vectors show however that basal shear cannot be the exclusive driving force of plate tectonics and suggest force contributions of comparable magnitude for perhaps half of the plates from the lithosphere itself specifically subducting slab pull and continental collision drag which are provisionally evaluated the relationship of the putative mantle benard polygons to dynamic chaos and turbulent flow is discussed

patterns of hot spot volcanism the aim of this paper is to identify the characteristic features in terms of geophysically measured parameters of the hot spot sites over the earth s surface we use a pattern recognition approach which identifies associations as well as single parameters and our statistical algorithm allows us to work at a given significance level avoiding any overfit we use the following parameters absolute plate velocity of the site minimum distance between neighboring hot spots minimum distance from a ridge minimum distance from a trench geoid anomaly of harmonic degrees 2 10 and 11 36 area of the host plate lithospheric thickness and stress state the hot spots generally defined as volcanoes not obviously related to plate boundaries are found to occur in sites characterized by either one of two main significant patterns 1 positive values of geoid anomalies with harmonic degree between 2 and 10 which is characteristic of most of the hot spots with a track of extinct volcanoes 2 clustered volcanoes distance between volcanoes less than 900 km in slow moving plates velocity 2 5 cm yr a third pattern representative of isolated volcanoes distance between volcanoes greater than 900 km anomalously close to mid ocean ridges distance from the ridge less than 600 km can also be tentatively identified this result implies the existence of two main types of hot spot volcanism the first one of sublithospheric origin with dynamics which is not influenced by surface tectonics and the second one mainly due to a favorably soft lithosphere a third type would probably consist of anomalous and magmatically very productive parts of mid ocean ridges

models for the hotspot distribution published hotspot catalogues all show a hemispheric concentration beyond what can be expected by chance cumulative distributions about the center of concentration are described by a power law with a fractal dimension closer to 1 than 2 random sets of the corresponding sizes do not show this effect a simple shift of the random sets away from a point would produce distributions similar to those of hotspot sets the possible relation of the hotspots to the locations of ridges and subduction zones is tested using large sets of randomly generated points to estimate areas within given distances of the plate boundaries the probability of finding the observed number of hotspots within 10 of the ridges is about what is expected

efforts to monitor and characterize the recent increasing seismicity in central oklahoma abstract the sharp increase in seismicity over a broad region of central oklahoma has raised concerns regarding the source of the activity and its potential hazard to local communities and energy industry infrastructure efforts to monitor and characterize the earthquake sequences in central oklahoma are reviewed since early 2010 numerous organizations have deployed temporary portable seismic stations in central oklahoma to record the evolving seismicity a multiple event relocation method is applied to produce a catalog of central oklahoma earthquakes from late 2009 into early 2015 regional moment tensor rmt source parameters were determined for the largest and best recorded earthquakes combining rmt results with relocated seismicity enabled determination of the length depth and style of faulting occurring on reactivated subsurface fault systems it was found that the majority of earthquakes occur on near vertical optimally oriented northeast southwest and northwest southeast strike slip fault

on the applicability of thermal conduction models to mid plate volcanism comments on a paper by gass et al the proposition that mid plate hot spot magmatism is strongly discouraged by a combination of thick lithosphere and rapid plate motion is examined empirically on a global basis and found to be inadequate although hot spots perhaps tend to avoid cratonic areas the nonrandom worldwide distribution of hot spots appears primarily to reflect the state of the sublithospheric mantle plate thickness or speed is of lesser importance the high level of african mid plate volcanism since 25 m y b p appears to be part of a global pattern controlled by events in the mantle rather than a result of the africa plate coming to rest over sublithospheric thermal sources about 40 m y b p

plate kinematic constraints from mantle benard convection general kinematic implications for plate tectonics are determined for rayleigh benard convection of the mantle the continuum of all possible configurations of benard polygons is probed by large random samples of global configurations 450 000 to 54 000 000 for each of which the euler poles are determined on the basis of viscous coupling across the asthenosphere two computationally related methods lead first to euler pole restrictions for fourteen plates and second to restrictions on the benard cell configuration result no 1 euler poles occur in global preference patterns which are determined exclusively by the shape of the plate the observational hs2 nuvel1 model poles occur near regions preferred by benard convection eurasia excluded the agreement is best for the most accurate observational poles result no 2 seven specific mantle benard cells are indicated by present day plate motions the upwelling centers correlate with hotspot domains the major global subduction zones correlate with benard model downwelling this result is independent of the euler pole accuracy used in its determination and is consistent with the distribution of low seismic p wave propagation velocities determined by tomography and with shear wave splitting analysis within the asthenosphere conclusions the results suggest that the bulk mantle is divided into less than ten benard convection cells globally cf fohlmeister and renka 2002 each of which extends from the asthenosphere to the core mantle boundary turbulent flow and other perturbations of the benard kinematics appear to be limited these primally poloidal flow kinematics provide basal shear forces as a major component in driving plate tectonics and are specifically configured for the directions of plate motions the benard model is incomplete without a dynamic contribution from the lithosphere which represents a separate convection layer of the distinct polar kinematics of rigid plates the complete hybrid mechanism for driving plate tectonics includes lithospheric buoyancy dynamics specifically from the subducting pacific plate slabs to compensate for plate slowing due to the back flow sector of the hawaiian convection cell and collision drag dynamics principally for smaller plates or continental margins

geodynamical implications from the correlation of surface geology and seismic tomographic structure abstract we have investigated the relationship between the lateral seismic structure in the mantle and the geological features associated with major mantle upwellings the oceanic ridge system and hotspots using the technique of correlation analysis we have identified that these features extend quite deeply into the mantle to a depth of around 1300 km this finding is surprising for the ridges because it indicates that the style of upwelling is rather active and is somewhat two dimensional in character the confidence level of the correlation between ridges and slow seismic regions is maintained at a level of greater than 90 to a depth of 1300 km this is followed by a sharp decrease to 50 at 1700 km a similar correlation pattern is found for the hotspots in both cases the correlation maxima lie close to a depth of 1000 km where remnants of old slabs have recently been revealed by correlation analysis these findings suggest that dynamical processes at a depth of around 1000 km can be linked with surface tectonic activity this correlation disappears at a depth of around 1500 1700 km certain features of the surface tectonic pattern can again be recognized around a depth of 2000 km and in the deepest mantle where major hotspot plumes and the remnant old lithosphere are the dominant features there may be some dynamical relationship between the ridge system and the two megaplumes we have proposed a physical mechanism based on viscous heating within megaplumes to explain the two peaks in the correlation between the hotspots and the seismic heterogeneities

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

periodic hotspot distribution and small scale convection in the upper mantle hotspots are commonly attributed to mantle plumes and to a first order the hotspot distribution is correlated with long wavelength geoid anomalies and with higher temperatures in the lower mantle suggesting that the plumes are generated in the lower mantle i show that the second order distribution is periodic with a typical spacing of 800 1000 km such a spacing would be too narrow for plumes to ascend directly from the lower mantle but is close to the thickness of the upper mantle the alignment of convection cells in the upper mantle seems to control the periodicity to reconcile the first a and second order distributions assuming limited mass fluxes across the 670 km boundary a lateral deflection of plumes as they breach the transition zone from below is required

lightning remagnetization of the vredefort impact crater no evidence for impact generated magnetic fields 1 the vredefort impact crater in south africa is one of the oldest and largest craters on earth making it a unique analog for planetary basins intense and randomly oriented remanent magnetization observed in surface samples at vredefort has been attributed to impact generated magnetic fields this possibility has major implications for extraterrestrial paleomagnetism since impact generated fields have been proposed as a key alternative to the dynamo hypothesis for magnetization on the moon and asteroids furthermore the presence of single domain magnetite found along shock generated planar deformation features in vredefort granites has been widely attributed to the 2 02 ga impact event an alternative hypothesis is that the unusual magnetization and or rock magnetic properties of vredefort rocks are the products of recent lightning strikes lightning and impact generated fields can be distinguished by measuring samples collected from below the present surface here we present a paleomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes we show that the magnetization at depth is consistent with a thermoremanent magnetization acquired in the local geomagnetic field following the impact while random intense magnetization and some of the unusual rock magnetic properties observed in surface rocks are superficial phenomena produced by lightning because vredefort is the only terrestrial crater that has been proposed to contain records of impact generated fields this removes a key piece of evidence in support of the hypothesis that paleomagnetism of the moon and other extraterrestrial bodies is the product of impacts rather than past core dynamos

u pb geochronology and sr nd isotope compositions of groundmass perovskite from the newly discovered jurassic chidliak kimberlite field baffin island canada abstract we report the u pb age and sr nd isotope composition for perovskite isolated from forty six kimberlite samples located in the newly discovered chidliak field on baffin island canada the minimum duration of kimberlite magmatism in this field was 17 9 m y from 157 0 to 139 1 ma and represents a new jurassic kimberlite field in ne canada the most prolific period of kimberlite magmatism occurred between 152 and 142 ma 80 of dated kimberlites kimberlitic perovskite from these intrusions display a range in 87 sr 86 sr 0 7043 to 0 7030 and e nd t values 3 9 to 0 4 overlapping the isotopic field previously defined for southern african group i kimberlites the ages and isotopic compositions obtained for chidliak magmatism are identical to a number of jurassic kimberlite fields in eastern north america and sw greenland some of this jurassic kimberlite magmatism has a link to one or more mantle plume hotspot tracks but the chidliak kimberlites have an origin in the deep subcontinental lithospheric mantle and are part of a jurassic magmatic province that erupted along both margins of davis strait linked to upwelling asthenosphere continental rifting and mesozoic cenozoic development of oceanic crust in the labrador sea basin in contrast the location of other eastern north american jurassic kimberlites when plotted on a jurassic continental reconstruction aligns closely to the northernmost projection of contours 2 4 of the african large low shearwave velocity province consistent with a link to mantle plumes derived from the african mantle plume generating zone

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

capture of the canary mantle plume material by the gibraltar arc mantle wedge during slab rollback summary recent evidence suggests that a portion of the canary plume travelled northeastwards below the lithosphere of the atlas mountains in north africa towards the alboran domain and was captured 10ma ago by the gibraltar subduction system in the western mediterranean the capture would have been associated with the mantle return flow induced by the westwardretreating slab that would have dragged and trapped a portion of the plume material in the mantle wedge of the gibraltar subduction zone such material eventually contaminated the subduction related volcanism in the alboran region in this work we use scaled analogue models of slab plume interaction to investigate the plausibility of the plume capture an upper mantle scaled model combines a narrow 400 km edge fixed subduction plate with a laterally offset compositional plume the subduction dominated by slab rollback and toroidal mantle flow is seen to increasingly impact on the plume dynamics as the area of influence of the toroidal flow cells at the surface is up to 500 1350 km 2 while the plume head initially spreads axisymmetrically it starts being distorted parallel to the plate in the direction of the trench as the slab trench approaches the plume edge at a separation distance of about 500 km before getting dragged towards mantle wedge when applied to the canary plume gibraltar subductionsystem ourmodelsupportstheobservationallybasedconceptualmodelthatmantle plume material may have been dragged towards the mantle wedge by slab rollback induced toroidal mantle flow using a scaling argument for the spreading of a gravity current within a channel we also show that more than 1500 km of plume propagation in the sublithospheric atlas corridor is dynamically plausible

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

antipodal hotspots and bipolar catastrophes were oceanic large body impacts the cause abstract one aspect of the hotspot distribution that has received little attention is its antipodal character of 45 primary hotspots found in most hotspot compilations 22 49 form antipodal pairs within observed hotspot drift limits 20 mm yr in addition the available ages or possible age ranges for both hotspots of an antipodal pair tend to be similar 10 myr difference or overlap monte carlo simulations indicate that the antipodal primary hotspots locations and ages are not due to chance at the 99 confidence level p

earthquake hypocenters and focal mechanisms in central oklahoma reveal a complex system of reactivated subsurface strike slip faulting the sharp increase in seismicity over a broad region of central oklahoma has raised concern regarding the source of the activity and its potential hazard to local communities and energy industry infrastructure since early 2010 numerous organizations have deployed temporary portable seismic stations in central oklahoma in order to record the evolving seismicity in this study we apply a multiple event relocation method to produce a catalog of 3639 central oklahoma earthquakes from late 2009 through 2014 regional moment tensor rmt source parameters were determined for 195 of the largest and best recorded earthquakes combining rmt results with relocated seismicity enabled us to determine the length depth and style of faulting occurring on reactivated subsurface fault systems results show that the majority of earthquakes occur on near vertical optimally oriented ne sw and nw se strike slip faults in the shallow crystalline basement these are necessary first order observations required to assess the potential hazards of individual faults in oklahoma

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

modulation of mantle plumes and heat flow at the core mantle boundary by plate scale flow results from laboratory experiments we report results from analog laboratory experiments in which a large scale flow is imposed upon natural convection from a hot boundary layer at the base of a large tank of corn syrup the experiments show that the subdivision of the convective flow into four regions provides a reasonable conceptual framework for interpreting the effects of large scale flow on plumes region i includes the area of the hot thermal boundary layer tbl that is thinned by the large scale flow thereby suppressing plumes region ii encompasses the critically unstable boundary layer where plumes form region iii is the area above the boundary layer that is devoid of plumes region iv comprises the area of hot upwelling and plume conduits quantitative analysis of our experiments results in a scaling law for heat flux from the hot boundary and for the spatial extent of plume suppression when applied to the earth s core mantle boundary cmb our results suggest that large scale mantle flow due to sinking lithospheric plates can locally thin the tbl and suppress plume formation over large fractions of the cmb approximately 30 of heat flow from the core may be due to increased heat flux from plate scale flow furthermore cmb heat flux is non uniformly distributed along the cmb with large areas where heat flux is increased on average by a factor of 2 as a consequence the convective flow pattern in the outer core may be affected by cmb heat flux heterogeneity and sensitive to changes in plate scale mantle flow becauseofplumesuppressionanddfocusingtofhotmantlefromthecmbintozonesofupwellingflow plumeconduits hotspots

at least three scales of convection in a mantle with strongly temperature dependent viscosity abstract we studied the convective patterns developing at high rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature dependent viscosity within this sluggish lid regime three different scales of convection develop the largest convective scale is cellular with cold downwelling sheets of viscous fluid encasing hotter less viscous parts of the tank within each of those cells develop several typically 3 7 hot 3d upwelling plumes upon impinging under the cold thermal boundary layer each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank applying those results to the earth s mantle we suggest that the large scale features of mantle convection and the co existence of several scales of convection i e slabs and plumes are produced by thermal convection in a mantle with high rayleigh number and a strongly temperature dependent viscosity material this generates a convective pattern in two large scale cells the pacific and the indo atlantic boxes our experiments further suggest that what has been named the two hot superplumes i e the two seismically slow regions encased within the subduction rings are in fact each constituted of several hot instabilities moreover plumes impacts under the lithosphere should be surrounded by cold rings of small extent the asthenosphere appears therefore as the graveyard of both small scale convection and hot plumes generated in the system