the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

evolution of midplate hotspot swells numerical solutions the evolution of midplate hotspot swells on an oceanic plate moving over a hot upwelling mantle plume is amenable to numerical simulation in our model the plume supplies to the lithosphere a gaussian shaped thermal perturbation and dynamic support dominated by thermal buoyancy we consider the two fundamental mechanisms of transferring heat conduction and convection during the interaction of the lithosphere with the mantle plume the transient heat transfer equations with boundary conditions varying in both time and space are solved in cylindrical coordinates using the alternating direction implicit finite difference method on a 100 100 grid the topography geoid anomaly and heat flow anomaly of the hawaiian swell and the bermuda rise are used to constrain the models our results confirm the conclusion from previous work that the hawaiian swell can not be explained by conductive heating alone even if an extremely high thermal perturbation is allowed on the other hand the model of convective thinning predicts successfully the topography geoid anomaly and the heat flow anomaly around the hawaiian islands as well as the changes in the topography and anomalous heat flow along the hawaiian volcanic chain the model constrains the hawaiian plume to have a half wavelength of about 500 km a center heat flux 5 6 times the background value and a convective current velocity 3 10 times that of the background convective current comparatively the mantle plume for the bermuda rise is much weaker conductive heating is probably the dominant mechanism for the evolution of the bermuda rise

double flood basalts and plume head separation at the 660 kilometer discontinuity several of the world s flood basalt provinces display two distinct times of major eruptions separated by between 20 million and 90 million years these double flood basalts may occur because a starting mantle plume head can separate from its trailing conduit upon passing the interface between the upper mantle and the lower mantle this detached plume head eventually triggers the first flood basalt event the remaining conduit forms a new plume head which causes the second eruptive event the second plume head is predicted to arrive at the lithosphere at least 10 million years after the first plume head in general agreement with observations regarding double flood basalts

large topographic rises on venus implications for mantle upwelling topographic rises on venus have been identified that are interpreted to be the surface manifestation of mantle upwellings these features are classified into groups based on their dominant morphology atla and beta regiones are classified as rift dominated dione western eistla bell and imdr regiones as volcano dominated and themis eastern eistla and central eistla regiones as corona dominated at several topographic rises geologic indicators were identified that may provide evidence of uplifted topography e g volcanic flow features trending upslope we assessed the minimum contribution of volcanic construction to the topography of each rise which in general represents less than 5 of the volume of the rise similar to the volumes of edifices at terrestrial hotspot swells the total melt volume at each rise is approximated to be 104 106 km3 the variations in morphology topography and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling instead the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure plume characteristics and regional tectonic environment within each class variations in topography gravity and amount of volcanism may be indicative of differing stages of evolution the similarity between swell and volcanic volumes for terrestrial and venusian hotspots implies comparable time integrated plume strengths for individual upwellings on the two planets

formation and growth of deep mantle plumes summary if whole mantle convection occurs in the earth s mantle then the core mantle boundary constitutes the lower boundary layer for mantle convection this boundary layer appears to be unstable on a small scale and thus may be a source of plumes of hot matter which penetrate the mantle and occasionally even the lithosphere producing hot spots a finite amplitude numerical code is used to study the formation of such plumes and their growth through the mantle the plumes are restricted to being two dimensional sheets rather than cylinders the initial conditions consist of a steadily convecting mantle and plumes are produced by introducing a perturbation in the form of either a pulse or a steady stream of heat into the bottom of the mantle two main results are obtained 1 a critical perturbation size has been found for a mantle with a rayleigh number of lo small perturbations produce plumes which fail to penetrate the mantle and instead are swept up by the pre existing convective pattern while large perturbations succeed in penetrating the mantle and reaching the lithosphere the critical perturbation size is shown empirically to be proportional to the effective bouyancy and to a factor related to the shape of the perturbation a perturbed region 150km wide and 60 km deep should produce a successful plume when the temperature perturbation is 200k or more 2 deep mantle plumes appear to require on the order of 50 100myr to penetrate the mantle episodic plumes on shorter time scales appear unlikely a similar time is required for plumes forming in an initially static uniform temperature mantle

abandoned penguin rookeries as holocene paleoclimatic indicators in antarctica penguins are sensitive indicators of the antarctic climate and of the environmental parameters that limit their presence and distribution paleoenvironmental data obtained from the study of abandoned penguin rookeries pygoscelis adeliae along the victoria land coast in antarctica indicate 14c date of 11 13 ka for the oldest abandoned rookery and supply new information about the timing of glacier retreat in southern victoria land after the last glacial maximum the continuous presence of the adelie penguins is documented from 7 ka according to our data the limiting factors that control the presence of penguins along the coast of victoria land changed during the holocene whereas several colonies were occupied for very long periods other sites were used for more or less extended periods and then abandoned the greatest diffusion of rookeries occurred between 3 and 4 ka a period of particularly favorable environmental conditions that has never been repeated it was followed by a sudden decrease in the number of penguin rookeries shortly after 3 ka this event has been attributed to an increase of the sea ice extension and may have been correlated to a worldwide phase of climate change near the subboreal subatlantic boundary a minor phase of penguin reoccupation occurred locally in the eighth to fourteenth centuries a d because the presence and number of penguins reflect the state of health of the antarctic marine ecosystem it is important to evaluate the variations in their distribution in the past in the absence of human induced changes

coupling surface and mantle dynamics a novel experimental approach recent modeling shows that surface processes such as erosion and deposition may drive the deformation of the earth s surface interfering with deeper crustal and mantle signals to investigate the coupling between the surface and deep process we designed a three dimensional laboratory apparatus to analyze the role of erosion and sedimentation triggered by deep mantle instability the setup is constituted and scaled down to natural gravity field using a thin viscous sheet model with mantle and lithosphere simulated by newtonian viscous glucose syrup and silicon putty respectively the surface process is simulated assuming a simple erosion law producing the downhill flow of a thin viscous material away from high topography the deep mantle upwelling is triggered by the rise of a buoyant sphere the results of these models along with the parametric analysis show how surface processes influence uplift velocity and topography signals

retreat of northern margins of george vi and wilkins ice shelves antarctic peninsula the george vi and wilkins ice shelves are considered at risk of disinteg ration due to a regional atmospheric warming trend on the antarctic peninsula retreat of the northern margin of the george vi ice shelf has been observed previously but the wilkins ice shelf was thought to be stable we investigated the positions of the northern fronts of these shelves from the literature and looked for changes on 1974 landsat and 1992 and 1995 european remote sensing satellite ers synthetic aperture radar images our investigation shows that the northern george vi ice shelf lost a total of 906 km 2 between 1974 and 1992 and an additional 87 km2 by 1995 the northern margin of the wilkins ice shelf lost 796 km2 between 1990 and 1992 and another 564 km2 between 1992 and 1995 armadas of tabular icebergs were visible in front of this shelf in the ers images these two ice shelves mark the southern most documented conspicuous retreat of ice shelf margins

time dependent convection in the earth s mantle abstract the range of rayleigh numbers accessible to 2 dimensional numerical models of mantle convection has been increased by almost two orders of magnitude through the use of high resolution numerical grids with up to 200 intervals in each of the horizontal and vertical directions the range of rayleigh numbers studied is believed to encompass conditions which would have existed in a hot archaean mantle although the models are highly idealized in that they neglect the compressibility and variations of viscosity in the earth s mantle they suggest that during the archaean and earlier boundary layer instabilities developed more frequently than at present and did so with a scale and separation comparable to the thickness of the thin thermal boundary layers rather than to the depth of the convecting layer itself the rising and sinking plumes which develop from these instabilities drive transient flows which nevertheless have a horizontal scale comparable to the depth of the convecting layer thus although surface motions during the archaean may have been more variable than at present in terms of speed and direction they might have occurred on a horizontal scale which is similar to that of present day plate motion the numerical models indicate that motions at the upper surface of time dependent convection cells are 1 predominantly controlled by transient sinking plumes and 2 converge directly towards the source region of the sinking plumes on the other hand rising plumes have a more diffuse influence on the upper surface and surface divergence does not always occur immediately above a rising plume if oceanic plates on earth constitute the upper surface of mantle wide convection cells the more pronounced geophysical signature of subduction zones compared to oceanic ridges may be simply due to the fact that buoyancy is most concentrated at the earth s surface in the form of sinking plumes the total surface heat flow is found to fluctuate significantly in the time dependent convection models since estimates of the present day heat loss from the earth s interior are at best instantaneous measurements it is possible that these estimates are not representative in a time averaged sense of the present thermal state of the planetary interior

corona associations and their implications for venus abstract geologic mapping principles were applied to determine genetic relations between coronae and surrounding geomorphologic features within two study areas in order to better understand venusian coronae the study areas contain coronae in a cluster versus a contrasting chain and are 1 directly west of phoebe regio quadrangle v 40 centered at latitude 15 s longitude 250 and 2 west of asteria and beta regiones between latitude 23 n longitude 239 and latitude 43 n longitude 275 results of this research indicate two groups of coronae on venus 1 those that are older and nearly coeval with regional plains and occur globally and 2 those that are younger and occur between beta atla and themis regiones or along extensional rifts elsewhere sometimes showing systematic age progressions mapping relations and earth analogs suggest that older plains coronae may be related to a near global resurfacing event perhaps initiated by a mantle superplume or plumes younger coronae of this study that show age progression may be related to 1 a tectonic junction of connecting rifts resulting from local mantle upwelling and spread of a quasi stationary hotspot plume and 2 localized spread of post plains volcanism we postulate that on venus most of the young post resurfacing coronal plumes may be concentrated within an area defined by the bounds of beta atla and themis regiones

honeycomb development on alexander island glacial history of george vi sound and palaeoclimatic implications two step cliffs mars oasis w antarctica abstract analysis of three generations of glacial deposits and of a range of geomorphic features including widespread honeycombs and tafonis at two step cliffs mars oasis 71 52 s 68 15 w provides new insights into the geomorphological evolution of west antarctica with special respect to alveolar weathering at two step terrace indicators of the inherited character of cavernous weathering were found such as 97 non flaking and varnished backwalls and 80 tafoni floors that are till covered and or sealed by lithobiontic coatings based on the ne predominant aspect of the alveolized boulder faces tafoni initiation is attributed to coastal salt spray weathering by halite coming from the george vi sound during the 6 5 ka bp open water period the present day activity of these inherited cavities is restricted to roof flaking attributed to a combination of processes involving thermal stresses this 6 5 ka bp phase of coastal alveolization is the first step of a six stage holocene geomorphological scenario that includes alternatively phases of glacial advance or stationing and phases of vegetal colonization and or rock weathering and aeolian abrasion on the deglaciated outcrops this geomorphic scenario is tentatively correlated with the available palaeoenvironmental record in the antarctic peninsula region with two potential geomorphic indicators of the holocene optimum being identified 1 clusters of centimetric honeycombs facing the sound marine optimum at 6 5 ka bp 2 salmon pink lithobiontic coatings preserved inside cavities and at the boulder surface terrestrial optimum at 4 3 ka bp

a gravity current model of cooling mantle plume heads with temperature dependent buoyancy and viscosity gravity currents are a ubiquitous fluid dynamical phenomenon which involve the horizontal spreading of fluid masses under their own weight or buoyancy a theoretical model is developed to account for the effects of bulk cooling on the dynamics and morphology of geological gravity currents with particular focus on mantle plume heads spreading beneath the lithosphere as many geological gravity currents e g plume heads and lava flows spread they cool and thereby become more viscous and dense all gravity currents initially spread at the same rate as the isothermal currents predicted by huppert 1982 however currents with temperature dependent viscosity and or buoyancy eventually go to a much slower spreading rate than the isothermal currents moreover unlike the isothermal gravity currents cooling variable viscosity and or variable buoyancy currents do not conserve shape as they spread both constant volume and constant volume flux currents with strongly temperature dependent viscosity develop steep sided flat topped plateau shapes which become more rounded once the currents lose most of their heat currents with temperature dependent buoyancy develop inflections or even extensive swelling at their flow fronts the surface expression of the edge steepening effect in mantle plume heads is likely to be filtered by lithospheric flexure but may contribute to the flattened plateau shape inferred by wessel 1993 for the hawaiian swell the frontal inflation effect due to variable buoyancy may contribute to the dual lobe structure of the hawaiian swell gravity anomaly and suggests an alternate physical mechanism for forming the torus or horseshoe shaped geochemical patterns in the galapagos and the marquesas hotspots perhaps most significantly the gravity current model also predicts the thermal i e degree of melting pattern for galapagos and marquesas hotspots more readily than the traditional entrainment models

a thermal model for the origin of post erosional alkalic lava hawaii abstract the model of lithospheric thinning and reheating for the origin of the hawaiian swell assumes that the lower lithosphere 60 km is rapidly reset to an asthenospheric temperature as it passes over the hot spot it is shown that this heat input induces melting in a few kilometer thick layer of lithosphere just above the thermal anomaly by solving the appropriate energy equation the mean degree of melting in the molten layer was estimated to be 1 5 with a total melt thickness of 25 150 m the minimum width of the thermal anomaly required to account for the observed rate of post erosional eruptions is of the order of 10 40 km which is probably satisfied the melt generated by this process matches the petrological and geochemical characteristics of hawaiian post erosional lava and their typical morb related isotopic signature because small degrees of melting are involved the extraction time scale is long a few million years and is consistent with the time span of post erosional eruptions also the characteristic sequence of hawaiian volcanism can be explained if the source for hawaiian lava is considered as a molten layer with melt fraction decreasing upward

isotopic evolution of lavas from haleakala crater hawaii pb and sr isotopic ratios have been determined for tholeiitic shield building alkalic cap and post erosional stage lavas from haleakala crater pb isotopic compositions of the tholeiites overlap those of the alkalic cap lavas although87sr 86sr ratios of these two suites are distinct alkalic cap and post erosional lavas appear to be indistinguishable on the basis of sr and pb isotopic composition sr and pb isotopic ratios of haleakala post shield building lavas are positively correlated such a trend is previously undocumented for any suite of hawaiian lavas and contrasts with the general negative correlation observed for data from hawaiian tholeiites these relations are consistent with a three component petrogenetic mixing model specifically it is proposed that magma batches at individual hawaiian volcanoes formed by 1 mixing of melts generated from mantle plumes containing two isotopically distinct mantle components primitive vs enriched and 2 subsequent variable degrees of interaction between these plume melts and a third morb signature mantle reservoir prior to their emplacement in a crustal magma chamber these observations and inferences provide new constraints on physical models of hawaiian magmatism based on observed temporal isotopic variations of haleakala lavas it is suggested that the ratio of enriched primitive mantle components in the hawaiian plume source decreases during the waning stages of alkalic volcanism over the same time interval both decreasing melt production and protracted residence of ascending melts within the upper mantle contribute to a systematic increase in the ratio of depleted vs plume component

viscosity and thickness of the sub lithospheric low viscosity zone constraints from geoid and depth over oceanic swells abstract the medium wavelength geoid to depth anomalies ratio gdr at oceanic hotspot swells has been found to increase from 0 5 m km to 5 m km according to the age of the lithosphere they occur on in order to interpret this trend the geoid and topography anomalies associated with mantle convective plumes crossing a sublithospheric low viscosity zone lvz have been derived from numerical models and a systematic investigation of the gdr dependence on the viscosity and depth extent of the lvz on the thickness and thermal structure of the lithosphere and on the rayleigh number has been conducted it is shown that for viscosity drops across the base of the lvz greater than one order of magnitude the gdr is strongly dependent on the depth of shallow interfaces such as the lithosphere athenosphere boundary and on the lvz s thickness consequently the empirical trend can be accounted for by the thickening of the lithosphere with age provided it occurs at the expense of a lvz whose base is at a fixed depth around 200 km in such a frame no significant variation with age of the lvz s viscosity is required by the gdr data best fit with the empirical trend is found for a lvz about 50 times less viscous than the underlying mantle the mantle flow starts to fluctuate when the local rayleigh number of the low viscosity layer exceeds the rayleigh number of the underlying mantle the fluctuations are initiated in the upper boundary layer in the diverging part of the plume at a distance of a few hundreds of kilometers from the main ascending current for viscosity contrasts in the range of 40 60 deduced from the present study the conditions for the development of these small scale instabilities are realized only where the lithosphere has not yet grown significantly downwards ages

numerical investigations of the mantle plume initiation model for flood basalt events recent studies have linked both the eruption of continental flood basalts and the emplacement of large oceanic plateaus to the initial phase of mantle plume activity i e the plume head model here we develop this concept further through numerical models of a large thermal diapir rising through the mantle and impinging upon the base of the lithosphere a finite element analysis in axisymmetric geometry is used to model the dynamics of solid state convective flow and heat transport in the mantle and an anhydrous batch melting model is used to estimate melt volumes and compositions obtained from partial melting in the shallow mantle we explore the effects of a number of model variables including mantle viscosity structure plume temperature and diameter and the mechanical response of the lithosphere our study emphasizes cases in which the lithosphere does not undergo significant extension prior to large scale melting a standard model is found to be consistent with some observed characteristics of flood basalt plume initiation events this model includes an initial diapir of radius 400 km and initial excess temperature 350oc rising through a mantle of viscosity 102 pa s and impinging beneath a model oceanic lithosphere 100 ma in age if lithospheric extension does not occur melting is almost entirely sublithospheric the melt volume produced is 3 5x106 km 3 and its composition has high mgo content 15 20 wt a time sequence of precursory central axial uplift of several kilometers followed by major melting volcanism is a robust feature of all the models regardless of mantle viscosity structure or lithospheric response low viscosity in the upper mantle compresses the timescale for these events and results in a constriction of the plume head initially impinging on the lithosphere the radial extent of the melting region about 500 km at the time of maximum melt production is only slightly affected by the upper mantle viscosity for nonrifting lithosphere no significant melt generation occurs for initial plume excess temperatures less than about 300oc the initial plume radius has only a modest effect upon the volume of melt obtained the most important aspect of lithospheric response is whether or not the uppermost crust and lithosphere above 40 km depth undergo extension otherwise the ductile strength of the lower lithosphere has little effect if the lithosphere is allowed to extend freely the melt volume increases by about an order of magnitude and it is comparable with some of the largest oceanic plateaus such as ontong java if continental or oceanic flood basalt events occur on old nonrifting lithosphere the primary melts are likely to be much more mgo rich than the basalts erupted at the surface this is consistent with abundant evidence for olivine fractionation in many flood basalt provinces along with the occurrence of picrite lavas and we hypothesize that fractionation occurs at a density trap at the crust mantle boundary moho at this stage and following it is likely that significant lower crustal and mantle lithospheric contamination of mantle plume derived magmas may occur both isotopically and in the trace elements the sequence and timing of both precursory and post magmatic events in the vast well exposed oceanic flood basalt terrane known as wrangellia se alaska british columbia are consistent with the predictions of our model the uplift predicted by our models is of order 2 4 km if we require that mantle plumes are hot enough to melt beneath unrifted lithosphere effects not modeled here such as small scale convection and hydrous melting will tend to reduce the model excess temperature and uplift in order to better understand the relation between mantle plumes and large igneous provinces several important advances are required implementation of a fractional melting model some realistic assessment of melt transport through the mantle and crust and a fully three dimensional convection model

insights into corona formation through statistical analyses 1 statistical analysis of an expanded database of coronae on venus indicates that the populations of type 1 with fracture annuli and 2 without fracture annuli corona diameters are statistically indistinguishable and therefore we have no basis for assuming different formation mechanisms analysis of the topography and diameters of coronae shows that coronae that are depressions rimmed depressions and domes tend to be significantly smaller than those that are plateaus rimmed plateaus or domes with surrounding rims this is consistent with the model of smrekar and stofan 1997 and inconsistent with predictions of the spreading drop model of koch and manga 1996 the diameter range for domes the initial stage of corona formation provides a broad constraint on the buoyancy of corona forming plumes coronae are only slightly more likely to be topographically raised than depressions with type 1 coronae most frequently occurring as rimmed depressions and type 2 coronae most frequently occurring with flat interiors and raised rims most type 1 coronae are located along chasmata systems or fracture belts while type 2 coronae are found predominantly as isolated features in the plains coronae at hot spot rises tend to be significantly larger than coronae in other settings consistent with a hotter upper mantle at hot spot rises and their active state

the interaction of mantle plumes with surface thermal and chemical boundary layers applications to hotspots on venus large volcanic swells on venus are believed to be a manifestation of mantle upwelling or hotspots the study of these regions provides important information on the interior of the planet numerical experiments are carried out to examine the interaction of mantle plumes with the thermal lithosphere and a layer of depleted mantle a product of pressure release melting using an axisymmetric finite element and finite difference code that incorporates temperature dependent viscosity and pressure release melting the lithosphere is defined as a high viscosity lid plumes are initiated and maintained by a prescribed temperature at the base of the computational domain the topographic uplift the geoid to topography ratio and the volume of pressure release melt are compared to estimated values for possible hotspots on venus to constrain the properties of plumes and the lithosphere the effects of lithospheric thickness depleted layer viscosity and thickness mantle temperature and plume temperature and duration on the surface observables are predicted models with a thermal lithospheric thickness of approximately 100 150 km are consistent with observations assuming a mantle temperature of 1300 c a maximum plume temperature of approximately 1500 c and mantle plume durations of 150 250 m y to be consistent with estimated volumes of volcanics at venusian hotspots a significantly thinner thermal lithosphere requires a much cooler mantle a thicker lithosphere requires a hotter mantle models with a depleted layer thickness of 100 250 km in addition to a 100 km thick thermal lithosphere predict the range of parameters found on venus interpretation of large volcanic swells on venus based on the evolutionary sequence predicted here implies that beta atla western eistla and imdr regiones overlie either active or recently active mantle plumes the geoid to topography ratio at beta regio is much larger than values found at other hotspots and may indicate that it is in an early stage of evolution or that the chemical or thermal boundary layers are significantly thicker bell dione and themis regiones appear to represent very late stage possibly now extinct hotspots an estimate of the total plume buoyancy flux for venus is much lower than the value obtained for earth the small areal distribution of volcanism is consistent with a low level of ongoing resurfacing at a few active hotspots

instabilities of fluid conduits in a flowing earth are plates lubricated by the asthenosphere summary a laboratory and theoretical study of the stability of conduits of buoyant fluid in a viscous shear flow has been conducted the object of the study is to explain the formation of discrete islands in island chains such as the hawaiian emperor seamount chain and to investigate a new method by which the variation of shear with depth in the mantle may be determined the conduits were made by injecting oil into a more viscous oil of greater density initially a growing chamber of lower viscosity oil formed near the injector but when the chamber got sufficiently large it rose as a buoyant spheroid behind this trailed a vertical cylindrical conduit through which fluid could continue to rise to the surface as long as the source continued if the more viscous fluid was sheared laterally the conduit was gradually rotated to a more horizontal position the diameter of the conduit increased with time due to a decreasing component of gravitational force along the axis of the conduit when the conduit was tilted to more than 60 with the vertical it began to go unstable by developing bumps which ultimately initiated a new chamber which rose to a new spot in addition if the reynolds number of the conduit was greater than approximately ten an axisymmetric wavy instability appeared in the walls of the conduit and the conduit had to be tilted less before a new chamber was initiated if shear under the pacific plate has to tilt buoyant mantle plumes to as much as 60 to form the relatively regular island chains associated with hot spots most of the shear would be found in a zone with a vertical extent of less than 200 km

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

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

a spreading drop model for plumes on venus many of the large scale plume related features on venus can be modeled by a buoyant viscous drop or plume head as it rises and spreads laterally below a free fluid surface the drop has arbitrary density and viscosity contrast and begins as a sphere below the surface of a fluid half space the boundary integral method is used to solve for the motion of the plume head and for the topography geoid and stress at the fluid surface as the plume approaches the surface stresses in the fluid above it cause it to spread and become thin below the surface during the spreading the surface swell above evolves through various stages whose morphologies resemble several different plume related features observed on venus when the plume head first approaches the surface a high broad topographic dome develops with a large geoid and radial extensional deformation patterns at later stages the topography subsides and becomes plateau like the geoid to topography ratio gtr decreases and the dominant stress pattern consists of a band of concentric extension surrounded by a band of concentric compression we find that a low viscosity model plume head viscosity that is 0 1 times the mantle viscosity produces maximum topography that is 20 lower and swell features which evolve faster than for an isoviscous plume we compare model results with both the large scale highland swells and smaller scale features such as coronae and novae the dome shaped highlands with large gtrs such as beta atla and western eistla regiones may be the result of early stage plume motion while the flatter highlands such as ovda and thetis regiones which have lower gtrs may be later stage features comparison of model results with gtr data indicates that the highlands result from plume heads with initial diameters of about 1000 km on a smaller scale an evolutionary sequence may begin with novae domes having radial extensional deformation followed by features with radial and concentric deformation such as arachnoids and end with coronae with mostly concentric deformation the model predicts that the highlands evolve on a timescale of order 10 ma and the smaller scale features evolve in a 100 ma timescale

late quaternary glacial history of george vi sound area west antarctica during the last glacial maximum in west antarctica separate ice caps developed on alexander island and on palmer land became confluent in george vi sound and discharged northward from latitude 72 s radiocarbon 32 000 yr and amino acid approximately 120 000 yr age determinations on shell fragments hiatella solida found in basal till suggest a wisconsin age for the glaciation that incorporated them the pattern of ice flow differed from that deduced for this area in the climap reconstruction following the maximum stage there was a stadial event when outlet valley glaciers flowed from smaller ice caps into george vi sound more widespread recession permitted the george vi ice shelf to deposit palmer land erratics on eastern alexander island before isostatic recovery raised them to final elevations of about 82 m the ice shelf may have been absent at about 6500 yr b p when large barnacles bathylasma corolliforme were living in the sound small glaciers readvanced to form at least two terminal moraines before the ice shelf re formed and incorporated the barnacle shells into its moraine on alexander island the shells gave a 14c age corrected for antarctic conditions of about 6500 yr b p and an amino acid ratio consistent with a holocene age valley glaciers readvanced over the ice shelf moraine before oscillations of both valley glaciers and the ice shelf led to the formation of the present sequence of contiguous ice cored moraines probably during the little ice age such oscillations may represent a climatic control not yet observed in the dry valleys of victoria land the only other part of antarctica studied in detail for glacier fluctuations

heat flow observations on the bermuda rise and thermal models of midplate swells the bermuda rise is a broad topographic swell which is apparent in both residual depth and geoid anomaly maps of the western north atlantic the magnitudes of the depth and geoid anomalies associated with the bermuda rise are similar to the anomalies associated with other swells surrounding recent volcanic islands e g hawaii suggesting that despite the lack of recent volcanism on bermuda the rise has a similar origin to other midplate swells results are reported from 171 new heat flow measurements at seven carefully selected sites on the bermuda rise and the surrounding seafloor off the bermuda rise the basement depths are generally shallower and the heat flow higher than either the plate or boundary layer models predict with the measured heat flow apparently reaching a uniform value of about 50 mw m 2 on 120 m y old crust on the bermuda rise the heat flow is significantly higher 57 4 2 6 mw m 2 than off the swell 49 5 1 7 mw m 2 the magnitude of the anomalous heat flux 8 10 mw m 2 is comparable to that previously found along the older portion of the hawaiian swell near midway the existence of higher heat flow on both the hawaiian swell and bermuda rise indicates that these features fundamentally have a thermal origin the differences in the shape uplift and subsidence histories of the hawaiian swell and bermuda rise can be quantitatively explained by the different absolute velocities of the pacific and north american plates moving across a distributed heat source in the underlying mantle two dimensional numerical convection models indicate that the observed depth geoid and heat flow anomalies are consistent with simple convection models in which the lower part of the thermally defined plate acts as the upper thermal boundary layer of the convection

global distribution and characteristics of coronae and related features on venus implications for origin and relation to mantle processes coronae on venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies the annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both topographically coronae are domes plateaus plateaus with interior lows and rimmed depressions a subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large scale calderas a number of features have been identified with many of the basic characteristics of coronae similar interior deformation associations with volcanism high topography but lacking a distinct tectonic annulus these features tend to be somewhat smaller than coronae and may represent failed coronae or coronae in an early stage of evolution the size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form n d kd where n is the number of coronae with maximum widths greater than d km and 3 05 the spatial distribution of coronae is not random the features are concentrated in a few groups and along several chains coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as western eistla regio the largest corona artemis is actually larger than several volcanic rises on venus coronae and volcanic rises appear to be surface manifestations of mantle plumes there is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on earth instead a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume the morphology and size distribution of coronae highlands and major shields suggest that mantle upwelling on venus operates either on several spatial scales with coronae representing smaller scale upflows and major volcanic rises representing larger convective upwellings or on several temporal scales with coronae representing shorter duration upflows and major volcanic rises representing long term upwellings

formation of seafloor swells by mantle plumes the prominent oceanic hot spots are located at the crest of broad topographic swells on the seafloor available evidence indicates that swells are supported by anomalously thin or altered lithosphere as might result from the intrusion of a mantle plume into the lithosphere a set of experiments have been made to measure the free surface uplift and subsidence produced by intrusion of a low viscosity buoyant plume into a rheological boundary layer a deep layer of strongly thermoviscous sugar solution with a free surface was cooled from above to simulate the lithosphere asthenosphere system low viscosity low density plumes were released at depth the free surface displacement generated by the ascending plume was recorded as a function of time using a proximity measurement system the plumes ascend through the asthenosphere as nearly spherical diapirs and collapse flatten upon intrusion into the base of the lithosphere evolution of the surface topography can be divided into three phases 1 a low axisymmetric rise supported by the plume deep beneath the lithosphere 2 rapid uplift and steepening as the plume reaches the lithosphere base and 3 slow axial subsidence as the plume stagnates within the lithosphere the measured uplifts are compared with the uplift calculated for buoyant plumes in a uniform viscous half space using morgan s image method the agreement is good for thin lithosphere for thick lithosphere the observed is less than the calculated uplift scaling laws have been derived which relate the measured maximum uplift and maximum uplift rate to diapir parameters extrapolation of these to the scale of ocean floor swells gives uplift rates of approximately 1 km in 5 m y by this mechanism the most significant qualitative results of the experiments are that swells formed by interaction of low viscosity mantle plumes and oceanic lithosphere can be characterized by axial symmetry rapid uplift and shallow compensation

impingement of mantle plumes on the lithosphere contrast between earth and venus the penetration ascent melt production cooling and solidification of a mantle plume as it impinges on the base of the lithosphere are modelled as functions of the lithospheric temperature and thickness the models indicate that if mantle and plume temperatures and plume dimensions of venus are similar to those of earth the higher lithospheric temperature of venus causes higher ascent of plumes into the lithosphere and a greater amount of lithospheric melting however the rates of intraplate volcanism on venus and earth appear to be similar indicating that mantle or plume temperatures may be less or plumes may be smaller in venus than in earth

variations in effective compensation depth across aphrodite terra venus aphrodite terra is the largest elevated terrain on venus and it serves as a focal point in current discussions of global tectonic style using the topography and gravity data acquired by the pioneer venus orbiter pvo the authors have estimated an effective depth of airy compensation for each of 75 orbital arcs that provide fairly uniform areal coverage of the entire province the most pronounced pattern that emerges is a general increase in compensation depth to the east the most rapid change occurs near 135 degree the average west of there is 70 km while the average to the east is 230 km superimposed on this larger trend are five distinctive regional patterns four well defined peaks and one interval of widely scattered and poorly constrained depths the maxima in compensation depth are well correlated with regional topographic highs while these observations are easily reconciled with the general notion that aphrodite is a region of crustal divergence the great depth of compensation is difficult to accord with the more specific suggestion that aphrodite is a terrestrial type divergent plate margin the alternative suggestion that aphrodite and the other equatorial highlands on venus are surface manifestations of hot upwelling mantle plumes ismore consistent both with the great depths of compensation and with the pattern of regional peaks and intervening troughs the broader scale increase in effective depth of compensation from west to east is more enigmatic but it might reflect an age progression of the plumes less

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

the origin of honeycomb weathering honeycomb weathering occurs throughout the world but the origin remains a matter of controversy wind erosion exfoliation frost shattering and salt weathering have been proposed as explanations although few attempts have been made to substantiate these hypotheses with chemical or mineralogical studies chemical analyses and field observations indicate that honeycomb weathering in coastal exposures of arkosic sandstone near bellingham washington results from evaporation of salt water deposited by wave splash microscopic examination of weathered surfaces show that erosion results from disaggregation of mineral grains rather than from chemical decomposition thin walls separating adjacent cavities seem to be due to protective effects of organic coatings produced by microscopic algae inhabiting the rock surface cavity walls are not reinforced by precipitation of elements released by weathering as has often been suggested at other locations honeycomb weathering develops rapidly and can be observed on surfaces that were planar less than a century ago

experimental and observational evidence for plume induced subduction on venus venus lacks plate tectonics but some trenches on venus resemble subduction zones laboratory experiments suggest that upwelling plumes can initiate localized subduction of a thin lithosphere such as the one on venus

an active thermoelastic rift mechanism finite element solutions for stresses and displacements during rift development are obtained for a variable thickness elastic lithosphere plate subjected to thermoelastic loading by conductive heating from an axial line source in the asthenosphere the asthenosphere is modelled as a fluid foundation the prerift lithosphere is modelled as an infinite continuous plate a semi infinite plate is used to characterize the rifted lithosphere horizontal restraints are removed at the rift axis to model the transition from a continuous to separated lithosphere plate during rifting lithosphere strength envelope curves derived from olivine flow laws are used to determine the elastic lithosphere depth in the solutions conductive heating of a variable thickness continuous lithosphere plate by a 2000 w m line source at 60 km depth produces high stresses oriented orthogonal to the forming rift axis and small vertical displacements the maximum uplift obtained in the continuous lithosphere plate models subjected to thermoelastic loading was 0 12 km at a distance of 120 km from the forming rift axis this small uplift is accompanied by shallow tensile stresses of 80 mpa at a distance of 5 km from forming rift axis which decrease to 6 mpa at a distance of 210 km shallow horizontal thermoelastic tensile stresses are over 5 times greater than the horizontal stresses produced by thermal buoyancy loads near the forming rift axis the transition from a continuous to rifted lithosphere is accompanied by an increase in lithosphere uplift at the rift axis from 0 11 to 0 42 km and a reduction in shallow horizontal stress magnitude by a factor of 30 thermoelastic loading may be a source of shallow tensile stress in the lithosphere and a contributing factor for the initiation of rifting thermal loading may also produce postrifting topography changes thermoelastic loading of the lithosphere is proposed as a possible driving mechanism for rift formation which produces separation and the transition from a continuous to rifted lithosphere plate

deflection of plumes by mantle shear flow experimental results and a simple theory summary linear hotspot tracks indicate relative motion between some hotspot sources and the overlying lithosphere in order to better understand the effects of plate motions and mantle convection on mantle plumes and associated hotspots we consider the path of a continuous plume of buoyant material in a shear flow laboratory experiments show that steady plumes bent over by a linear velocity profile follow parabolic trajectories plume trajectories are also shown to evolve quadratically in time toward a new steady shape after a change in the ambient flow a remarkably simple description of plume dynamics provides good agreement with the experimental data the plume trajectory is determined by the combination of horizontal advection by mantle flow and vertical rise according to a modified stokes law these results are applied to some idealised examples which demonstrate effects of depth dependent mantle viscosity and return flow at least some mantle plumes are likely to be sufficiently robust that they remain close to vertical and therefore stable

venus geology and tectonics hotspot and crustal spreading models and questions for the magellan mission spacecraft and ground based observations of venus have revealed a geologically young and active surface with volcanoes rift zones orogenic belts and evidence for hotspots and crustal spreading yet the processes responsible for these features cannot be identified from the available data the magellan spacecraft will acquire an unprecedented global data set which will provide a comprehensive and well resolved view of the planet this will permit global geological mapping an assessment of the style and relative importance of geological processes and will help in the understanding of links between the surface geology and mantle dynamics of this earth like planet

climate since ad 1510 on dyer plateau antarctic peninsula evidence for recent climate change a 480 year record of the oxygen isotope ratios dust content chemical species and net accumulation from ice cores drilled in 1989 90 on dyer plateau in the antarctic peninsula is presented the continuous analyses of small sub annual samples reveal well preserved annual variations in both sulfate content and 18o thus allowing an excellent time scale to be established this history reveals a recent pronounced warming in which the last two decades have been among the warmest in the last five centuries furthermore unlike in east antarctica on dyer plateau conditions appear to have been fairly normal from ad 1500 to 1850 with cooler conditions from 1850 to 1930 and a warming trend dominating since 1930 reconstructed annual layer thicknesses suggest an increase in net accumulation beginning early in the 19th century and continuing to the present this intuitive conflict between increasing net accumulation and depleted 18o cooler climate in the 19th century appears widespread in the peninsula region and challenges our understanding of the physical relationships among moisture sources air temperatures and snow accumulation the complex meteorological regime in the antarctic peninsula region complicates meaningful interpretation of proxy indicators and results in a strong imprint of local high frequency processes upon the larger scale climate picture

blob tectonics a prediction for western aphrodite terra venus it is proposed that the western aphrodite terra region of venus was formed by four diapirs of heated mantle material or blobs rising to the surface in a region of convective upwelling the four blobs are in different stages of evolution producing different topography gravity anomalies and morphology we show that this model is consistent with available data and make specific predictions that can be tested with magellan radar data

venusian highlands geoid to topography ratios and their implications abstract using pioneer venus line of sight gravity data and orbit simulation procedures we have estimated apparent depths of isostatic compensation adcs for twelve venusian highland features asteria atla bell beta ovda phoebe tellus thetis and ulfrun regiones and nokomis gula and sappho montes adcs range from 50 km to 270 km half of the values are less than 100 km using these adcs we estimate geoid to topography ratios gtrs for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots swells and plateaus the geoid is estimated in the wavenumber domain from the isostatic formula using the topography and adc for each region in the space domain the gtr is equal to the least squares slope of the linear fit of the geoid to the topography the resulting gtr range is 7 31 m km which is much higher than terrestrial oceanic values 1 to 5 m km the features fall into two distinct groups one with a gtr range of 7 13 m km and one with a range of 19 25 m km the exception is beta regio which has a gtr of 31 m km a model for thermal thinning of a 100 km thick lithosphere fits all values in the lower gtr group to within one standard deviation airy compensation could also be present but cannot fully compensate these features partial dynamic compensation of the lower gtr group in combination with lithospheric mechanisms is also possible but not required to fit the data the upper gtr range 19 25 m km can be fit with an upper mantle constant viscosity convection model the large gtr values are inconsistent with the presence of a low viscosity zone if more than one compensation mechanism is present in the regions in the higher gtr group the gtrs will be underestimated in terms of a dynamic interpretation we thus fit the convection models to the upper end of the gtr range 25 m km rayleigh numbers in the range 10 4 10 6 will produce a gtr of 25 m km when combined with conductive lid thicknesses of 85 150 km the 6 m km range in both of the gtr groups is probably due to varying degrees of crustal and thermal compensation combined with dynamic compensation in the upper gtr group the difference between terrestrial and venusian gtr ranges can be explained largely by the lack of a low viscosity zone on venus

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

impact origin for the greater ontong java plateau abstract the 120 ma ontong java plateau and neighboring contemporaneous nauru east mariana and probably pigafetta basin flood basalts in the western equatorial pacific ocean comprise the earth s largest flood basalt province geophysical geochemical and geodynamic evidence from the province are difficult to reconcile with mantle plume models absence of an obvious hotspot source or track minor crustal uplift associated with emplacement minor total subsidence compared with normal oceanic crust or other oceanic plateaus and submarine ridges high degrees of melting at shallow upper mantle depths low water contents of basalts enrichment of platinum group elements in basalts and a 300 km deep seismically slow mantle root are more consistent with the consequences of an impacting bolide an object 20 km in diameter impacting relatively young 20 myr pacific lithosphere and penetrating into the uppermost asthenosphere would have initiated massive decompression melting in the upper mantle and may have resulted in emplacement of the greater ontong java plateau

oceanographic and geophysical applications of satellite altimetry since the last quadrennial report new satellite altimeter data have not been generally available nevertheless important progress has been made from analyses of geos 3 and seasat data the marine gravity field has been determined to an extraordinary resolution and accuracy and important solid earth geophysical interpretations made in the area of oceanography statistical estimates of the variability of dynamic topography have been published methods for incorporating altimeter data into numerical models have been developed and applied and tentative but interesting determinations of the mean circulation have been made in addition altimeter waveform analysis has made it possible to use altimeter data to determine elevations of major ice fields finally important new insights into the nature of satellite ephemeris error and its influence on altimetrically derived sea surface topography have been achieved

linking lowermost mantle structure core mantle boundary heat flux and mantle plume formation abstract the dynamics of earth s lowermost mantle exert significant control on the formation of mantle plumes and the core mantle boundary cmb heat flux however it is not clear if and how the variation of cmb heat flux and mantle plume activity are related here we perform geodynamic model experiments that show how temporal variations in cmb heat flux and pulses of mantle plumes are related to morphologic changes of the thermochemical piles of large scale compositional heterogeneities in earth s lowermost mantle represented by the large low shear velocity provinces llsvps we find good correlation between the morphologic changes of the thermochemical piles and the time variation of cmb heat flux the morphology of the thermochemical piles is significantly altered during the initiation and ascent of strong mantle plumes and the changes in pile morphology cause variations in the local and the total cmb heat flux our modeling results indicate that plume induced episodic variations of cmb heat flux link geomagnetic superchrons to pulses of surface volcanism although the relative timing of these two phenomena remains problematic we also find that the density distribution in thermochemical piles is heterogeneous and that the piles are denser on average than the surrounding mantle when both thermal and chemical effects are included

effects of lithospheric properties on the formation of type 2 coronae on venus 1 over 500 coronae occur on venus a recent survey defined type 2 coronae as those with 50 complete fracture annuli this paper synthesizes the results from a statistical study of the size topographic morphology and geologic setting of the entire corona population and a study of the gravity and topography of the 32 type 2 coronae resolved in the gravity data using the estimated lithospheric properties loading signature and geologic characteristics we examine the factors controlling corona morphology size and fracture completeness type 1 versus type 2 we find no difference between lithospheric properties in the plains and fractures belts nearly half of the type 2 coronae studied are isostatically compensated and probably inactive this implies that the final stage of a corona may have a wide range of topographic morphologies elastic thickness has no correlation with diameter and therefore does not limit corona size the ratio of crustal thickness to plume diameter does not control morphology as a function of size as predicted by a spreading drop model however rim only coronae which are predicted to form via isostatic rebound of crust thickened by delamination are supported by a density interface isostasy and delamination of the lower lithosphere are likely to have a greater effect on corona formation and possibly in understanding the evolution of the venusian lithosphere than variations in elastic thickness the role of isostasy in the formation of type 2 coronae suggests that surface fractures may be reduced due to low strain rates

thermal origin of the hawaiian swell heat flow evidence and thermal models the lithospheric reheating hypothesis for the origin of the hawaiian swell has been tested by heat flow measurements along the swell between hawaii and midway ninety five measurements at eight carefully selected sites show that the heat flux on the swell increases systematically from near normal values 52 2 mw m 2 near hawaii to values near midway 59 2 mw m 2 about 20 25 higher than normal for lithosphere of that age 110 m y this heat flow pattern is qualitatively consistent with a deep seated thermal anomaly created in a lithospheric plate moving across a fixed heat source modeling of the heat flow and bathymetry data on the hawaiian swell strongly suggest that the swell is caused by a rapid heating within 40 to 50 km depth of normal 90 m y old lithosphere over a relatively broad 500 1000 km region near hawaii this model is also consistent with gravity and geoid anomalies associated with the swell which require a low density region in the lower lithosphere and with flexure studies of seamounts in the hawaiian chain which indicate that the rigidity of the lithosphere is not significantly reduced by the reheating appendices a c are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 005 1 00 payment must accompany order

the role of thermal stress fatigue in the breakdown of rock in cold regions abstract many geomorphological studies have ignored the role of thermal stress fatigue engineers have long maintained that thermal stress is an effective process physicists and the ceramics industry have provided a theoretical foundation to explain the nature and mode of operation of thermal breakdown recent geomorphological studies have shown the importance of thermal stress in cold regions by means of high frequency rock temperature data which can identify rates of temperature change in excess of 2 c min 1 the occurrence of weathered material in cold but dry regions can be explained by thermal stress shock differential radiation receipts on slopes of varying orientation and gradient can exert a very strong effect

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

a spatial domain stokes flow model for the gravity and topography of the middle latitudes of venus a new modeling strategy is developed and applied to the long wavelength gravity topography and internal density structure of venus geodynamic modeling of planetary scale phenomena is usually carried out in the wavenumber transform domain spherical harmonics or fourier transforms however because of limitations associated with the gravity and topography data available for venus a spatial domain approach is preferable smoothed representations of the gravity and topography of venus are developed in terms of quadratic b splines on 10 centers in latitude and longitude over the latitude interval from 45 s to 45 n these models reproduce 83 6 and 98 3 of their respective data variances a similarly constructed model is then presented for the dynamic support of gravity and topography on venus internal mass anomalies are specified at two discrete depths 50 and 500 km with lateral variations determined by two constraints 1 that the slow viscous flow induced by the anomalies will distort the free surface into a form matching the observed topography and 2 that the sum of the gravitational effects of the internal anomalies plus that due to the surface distortion they cause will match the observed gravity there are four major features evident in the gravity and topography data sets atla beta ovda and thetis regiones all four have compensating mass patterns consistent with a mantle plume origin the surficial flow pattern associated with this model does not resemble terrestrial plate tectonic patterns

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

rock thermal data at the grain scale applicability to granular disintegration in cold environments consideration of the mechanisms associated with the granular disintegration of rock has been limited by available data in most instances both the size of the transducer and the nature of the study have negated any applicability of the resulting data to the understanding of grain to grain separation within rock the application of microthermocouples 0 15 mm diameter and high frequency logging 20 s intervals at a taffoni site on southern alexander island and from a rock outcrop on adelaide island antarctica provide new data pertaining to the thermal conditions at the grain scale of the rock surface the results show that thermal changes t t can be very high with values of 22 c min 1 being recorded although available data indicate that there can be differences in frequency and magnitude of uctuations as a function of aspect all aspects experienced some large magnitude 2 c min 1 uctuations further in many instances large thermal changes in more than one direction could occur within 1 min or in subsequent minutes these data suggest that the surface grains experience rapidly changing stress elds that may with time effect fatigue at the grain boundaries albedo differences between grains and the resulting thermal variations are thought to exacerbate this the available data failed to show any indication of water freezing an exotherm and thus it is suggested that microgelivation may not play as large a role in granular breakdown as is often postulated for cold regions and that in this dry antarctic region thermal stress may play a signi cant role copyright 2003 john wiley sons ltd

a preliminary scanning electron microscope study of honeycomb weathering of sandstone in a coastal environment honeycomb weathering has been observed in a carboniferous sandstone at a coastal location near ballycastle on the north coast of northern ireland specimens of this sandstone have been analysed by x ray diffraction scanning electron microscopy and energy dispersive spectrometry results reveal that calcium sulphate gypsum is the only salt present and is found only at and immediately below the rock surface sem observations suggest that crystallization of salts in pores could easily dislodge quartz grains to promote granular disintegration whilst etching of quartz grain surfaces attests to chemical weathering activity within the rock however the reason for the development of the honeycomb pattern is not known

anomalous uplift and subsidence of the ontong java plateau inferred from co2 contents of submarine basaltic glasses the ontong java plateau in the western pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau paleoeruption depths at 122 ma estimated from dissolved h2o and co2 in submarine basaltic glass pillow rims vary from 1100 m below sea level mbsl on the central part of the plateau to 2200 3000 mbsl on the northeastern edge our results suggest maximum initial uplift for the plateau of 2500 3600 m above the surrounding seafloor and 1500 400 m of postemplacement subsidence since 122 ma our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere and thus our results are inconsistent with formation of the plateau by a high temperature mantle plume two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau 1 formed as a result of a giant bolide impact or 2 formed from a mantle plume but has a lower crust of dense garnet granulite and or eclogite neither of these possibilities is fully consistent with all available geological geophysical and geochemical data the origin of the largest magmatic event on earth in the past 200 m y thus remains an enigma

plagioclase and quartz preferred orientations in a low grade schist the roles of primary growth and plastic deformation albite in a lower greenschist facies haast schist from new zealand has a maximum of poles to 001 parallel to the elongation direction 010 forms a girdle about grain length plastic deformation with slip dominantly on 010 001 is rejected as the orienting mechanism because 1 the relationship of crystal axes to grain length would require multiple slip systems incompatible with the lack of rotation of albite twin planes 2 euhedral terminations and other textural features indicate primary crystallization 3 albites in quartzose layers are less elongate than those in sheet silicate rich layers the albite fabric is ascribed to growth anisotropy and competition in response to stress driven solution and growth during formation of metamorphic segregation layering this explanation is compatible with the common observation that plagioclase is brittle at low metamorphic grades associated weak quartz c axis fabrics are ascribed to plastic deformation and this deformation enhanced a dimensional preferred orientation already established by primary growth

geological evidence for a global tectonic polarity inferences about plate motions over the past 40 ma based on structural and geodetic data suggest that there is a coherent movement direction on a global scale moreover a westward drift of the lithosphere relative to the asthenosphere is indicated by plate motions in the hot spot reference frame independent geological observations such as the different styles of deformation in thrust belts associated with subduction following or opposing the relative eastward or northeastward mantle flow also support this global tectonic polarity unfortunately most hot spot reference frames are based on assumptions which strongly limit their validity for example many combine magmatic sources originating from different depths in the mantle including hot spots located along oceanic ridges which are not fixed relative to one another using a hot spot reference frame filtered to exclude shallow hot spots and anomalous wet spots we would predict a much higher relative westward drift of the lithosphere plate motions are not random rather that they are controlled by the relative motion between lithosphere and asthenosphere and by gradients in lithospheric thickness and composition triangular shaped plates may experience rotations resulting from differential drag at the base of the lithosphere with respect to the asthenosphere e g the clockwise rotation of south america plates appear to follow an undulatory pattern of motion on a global scale and their poles of rotation fall into a main cone located near the earth s rotation poles plate rotations induced by triangular shape differential basal drag or lithospheric anisotropies along subduction zones may generate the scattered distribution of euler poles this might have inhibited the recognition of a coherent global tectonic pattern

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

diapirism and topography summary a new numerical technique using markers and a deformable lagrangian mesh is used to study the deformation of the surface above a rising diapir this method allows modelling of a free surface boundary in a self consistent way the method makes it possible to investigate many different problems with non regular geometry the codes were verified through comparison with analytical solutions for the initial stages and with other numerical codes for mature stages our simulations led to the following conclusions 1 the growth rate of the diapir is more strongly influenced by the viscosity contrast than the wavelengths this is due to the strong growth rate difference during the initial stage 2 the maximum elevation above the diapir linearly increases with increasing wavelength and is approximately the same for different viscosity contrasts 3 the elevation will be considerably higher for a low viscosity diapir at a given depth than for an isoviscous diapir comparisons for different thicknesses of the buoyant layer showed that the highest topography is produced when the two layers have equal thickness we showed the dependence of the topographic behaviour on the parameter r the ratio of the density difference between two layers to the density of the upper layer it was found that topography behaves linearly up to r 0 15 it indicates that a posteriori estimation of topography for traditional calculations using the free slip boundary condition works well within this limit

heterogeneity in the lower mantle beneath africa as revealed from s and scs phases abstract s and scs phases propagated beneath africa and its adjacent regions were analyzed using their differential travel times t and the differences of anelastic parameter t between the two phases the residuals of the scs s differential travel times had strongly positive correlation with those of the scs travel times the largest residual of 7 9 s was in central africa 10 5 n 22 6 n the residuals decreased with increasing distance northwards from central africa the scs bounce points having large positive values greater than 4 s were concentrated in a region with a horizontal length of about 500 km at the core mantle boundary beneath central africa this region was characterized by an s wave velocity around 3 5 slower than prem in the d region the anelastic parameter t of the s phases propagated beneath africa was larger than that of the scs phases suggesting that the apparent q structure in the mid mantle 1000 2000 km deep was slightly lower than that at the base of the mantle these results probably imply a mantle plume beneath africa

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

iridium content of basaltic tuffs and enclosing black shales of the balder formation north sea abstract the anomalous levels of ir and the presence of shocked metamorphosed quartz deposited at the cretaceous tertiary k t boundary worldwide is strong evidence that a meteorite impact took place during the k t boundary interval however because of observed high ir contents at kilauea vents it is still a major point of contention that the ir anomaly could have been produced by flood basaltic volcanism this might especially be true at stevns klint denmark where the k t boundary marl contains pyroclastic labradorite and mg smectite thought to have been produced by basaltic volcanism however up to now no study has determined whether or not a depositional ir anomaly has formed in association with a known major basaltic eruption herein we report the concentrations of ir pt au and ag in basaltic tuffs and enclosing marine black shales of the widespread paleocene eocene balder formation the tuffs in the balder formation represent explosive basaltic volcanism associated with the major volcano tectonic activity of the opening of the northern north atlantic ocean as such they are the kind of eruption that could have possibly created a global k t boundary type ir anomaly our results show that the tuffs and the shales on a per weight basis both contain concentrations of ir 0 1 0 25 ppb that are higher than the ir levels recently measured from terrestrial rocks including the deccan trap and columbia river flood basalts but are comparable to hawaiian and reunion island basalts because of its thickness the absolute amount of ir expelled during the eruption of the main tuff sequence of the balder tuff is sizable yet for such an eruption to have produced a global ir anomaly would mandate it having been one of an extremely high volatile content and it would have to have been erupted over a very short interval of time furthermore such a high proportion of the volatilized ir would have to have been injected into the stratosphere so that only small enrichments of ir were formed in the proximal tuffs on a per gram basis

genesis of flood basalts and basin and range volcanic rocks from steens mountain to the malheur river gorge oregon the middle and south forks of the malheur river provide a unique mapping corridor connecting two flood basalt successions steens basalt to the south and the basalt of malheur gorge to the north each contains chemically defined subtypes which merge stratigraphically across the north south length of the map area the lowermost flows of steens basalt are stratigraphically equivalent to the lowermost flows of the basalt of malheur gorge the uppermost flows of steens basalt pinch out in the map area but are partly interbedded with the middle and uppermost flows of the basalt of malheur gorge which continue to thicken northward the upper part of the tholeiitic succession is interbedded with a group of previously unrecognized lavas the venator ranch basalt flows tholeiitic volcanism ceased at ca 15 3 ma the last tholeiitic unit erupted is the hunter creek basalt which also thickens northward subsequent younger than 15 3 ma more localized eruptions were dominated by calc alkaline to mildly alkaline lavas associated with basin and range extension local uplift generated deep canyons which were filled by andesitic lavas of the keeney sequence ca 13 10 ma the final eruptive products include the devine canyon tuff ca 9 7 ma the drinkwater basalt ca 6 9 ma and the voltage flow older than 32 000 yr b p major and trace element analyses demonstrate that 1 crystal fractionation was a universal process in the derivation of the

influence of peruvian flat subduction dynamics on the evolution of western amazonia convection in the earth s mantle is mainly driven by cold dense subducting slabs but relatively little is known about how 3d variations in slab morphology and buoyancy affect mantle flow or how the surface above deforms in response i e dynamic topography we investigate this problem by studying the dynamics of an active region of flat slab subduction located in peru in south america here the slab geometry is well known based on the regional seismicity and we have observations from the local geological record to validate our models of particular interest is the widespread subsidence and deposition of the solimoes formation across western amazonia that coincided with the development of the peruvian flat slab during the mid late miocene this formation covers an extensive area from the foredeep to the purus arch located view the mathml source 2000km away from the trench close to the andes the preservation of several kilometers of sedimentary thicknesses can be easily accounted for by flexure based on an estimate of the andean loading we predict 2 8 to 3 6 km of accommodation space that spans 100 km the spatial and temporal history of the solimoes formation however particularly the thick distal foreland accumulations up to 1 2 km deep can only be matched with the addition of a longer wavelength dynamic source of topography following the transition from normal to flat subduction we predict over 1 km of dynamic subsidence view the mathml source 1500km wide that propagates over 1000 km away from the trench tracking the subduction leading edge this is followed by a pulse of dynamic uplift over the flat segment behind it we therefore propose that a combination of uplift flexure and dynamic topography during slab flattening in peru is responsible for the sedimentation history and landscape evolution of western amazonia that eventually led to the configuration of the amazon drainage basin we know today

holocene glacier fluctuations in south america and antarctica abstract glacial geologic evidence indicates that glaciers throughout the andes and antarctica fluctuated during the holocene radiocarbon dating and other age determinations suggest that glaciers readvanced significantly only during the last 5 ka reaching positions from several 100 m to a few kilometres beyond their present limits in south america tenuous evidence from radiocarbon dates with dendrochronological data and environmental interpretations from pollen analyses indicate four main periods of neoglacial advance culminating 5000 4000 bp 3000 2000 bp 1300 1000 bp and 15th late 19th centuries smaller advances may have occurred at ca 8400 bp ca 7500 bp and ca 6300 bp the meagre data are consistent in indicating broad synchrony throughout the andes during the last 5 ka suggesting response to global climatic changes anomalies exist in patagonia where some tide water glaciers reached their maximal holocene limits recently this century the broad spectrum of differing antarctic environments produces interesting contrasts some local glaciers in the mcmurdo sound area of the east artarctic continent are more extensive now than during the global glacial maximum ca 18 ka bp when they were starved of precipitation consistent agreement among 14 radiocarbon dates from the south shetland islands indicates two main holocene glacier advances the most extensive 2 3 km peaking in the 12th century the other culminating in the 18th 19th century glaciers in south georgia reached their most advanced holocene limits before 2200 bp moraine fjord glacier which culminated as a 6 km advance between 1460 1700 radiocarbon bp may have lagged 400 650 years behind the climatic forcing because it could only advance in its deep water fjord by building a moraine bank smaller advances in south georgia culminated in the 17th 19th centuries and during the 1920s 30s there is no firm evidence of glacier advances before 3 ka bp in the southern ocean sub antarctic domain but broad synchrony in glacier advances during the last ca 3000 years appears to have occurred throughout the andes antarctic transect caution is required in the interpretation and correlation of moraines associated with calving glaciers and of those with poorly constrained dating

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

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

a geodynamic model linking cretaceous orogeny arc migration foreland dynamic subsidence and marine ingression in southern south america abstract this study synthesizes the tectonomagmatic evolution of the andes between 35 30 s to 48 s with the aim to spotlight early contractional phases on andean orogenic building and to analyze their potential driving processes we examine early tectonic stages of the different fold thrust belts that compose this andean segment additionally we analyzed the spatio temporal magmatic arc evolution as a proxy of dynamic changes in andean subduction during critical tectonic stages of orogenic construction this revision proposes a hypothesis related the existence of a continuous large scale flat subduction setting in cretaceous times with a similar size to the present largest flat slab setting on earth this potential process would have initiated diachronically in the late early cretaceous and achieved full development in late cretaceous to earliest paleocene times constructing a series of fold thrust belts on the retro arc zone from 35 30 s to 48 s moreover we assess major paleogeographic changes that took place during flat slab full development in maastrichtian danian times at this moment an enigmatic atlantic derived marine flooding covered the patagonian foreland reaching as far as the andean foothills based on flexural and dynamic topography analyses we suggest that focused dynamic subsidence at the edge of the flat slab may explain sudden marine ingression previously linked to continental tilting and orogenic loading during a high sea level global stage finally flat subduction destabilization could have triggered massive outpouring of synextensional intraplate volcanic rocks in southern south america and the arc retraction in late paleogene to early neogene times

holographic measurements of surface topography in laboratory models of mantle hotspots summary the evolution of surface topography produced by the rise of a buoyant droplet or diapir towards the free surface of a very viscous fluid in laboratory experiments is monitored using holographic interferometry such experiments enable us to investigate implications for surface topography of one possible unsteady model for intraplate hotspots the arrival of mantle thermals or diapirs near the base of the earth s lithosphere our model is possibly of most direct relevance to the interaction with the earth s surface and lithosphere of large spherical caps that are expected to rise at the head of new mantle plumes to previous experimental results for the axial height of topography olson nam we add further information on the height shape and width of the surface swell and on the evolution of the diapir itself when the ambient fluid has uniform density and viscosity no lithosphere surface topography is determined by the diameter density anomaly and depth of the droplet as the diapir approaches the surface a broad axisymmetric surface swell appears and initially increases in height while decreasing in width when the leading edge of the diapir is 0 2 diapir diameters below the surface the height passes through a maximum and the width through a minimum the swell then proceeds to subside and increase in width as the diapir spreads beneath the surface in separate experiments the lithosphere is modelled by a discrete surface layer of more viscous fluid whose thickness and viscosity contrast with the mantle are treated as independent parameters effects of lithosphere buoyancy relative to the mantle a property which may influence continental hotspot swells are also studied within the parameter range used the maximum swell height is independent of the lithosphere viscosity contrast but decreases with increasing lithosphere thickness and with decreasing lithosphere density surface uplift produced by the rise of two consecutive diapirs is shown to be more complex comparison of results with the characteristics of hotspot swells is attempted and should assist in determining the nature of hotspot plumes

rise of starting plumes through mantle of temperature pressure and stress dependent viscosity the evolution of size and excess temperature of starting plumes is calculated as a function of 1 newtonian and power law stress dependent mantle rheology and 2 buoyancy flux into the plume through a feeding conduit in mantle of temperature and pressure dependent viscosity the initial size and excess temperature that allow plumes starting at the core mantle boundary to reach the base of the lithosphere 200 k or more warmer than the surroundings are determined for a range of conditions the results suggest that starting plumes can be divided into slow and fast plumes slow plumes become large and cool through entrainment of the surroundings during ascent so that they are likely to have little impact on the evolution of the lithosphere fast plumes on the other hand rise so rapidly that cooling and entrainment become unimportant they reach the lithosphere while they are still hot often with an excess temperature similar to that at the source and are likely to strongly perturb the lithosphere the change from slow to fast plumes occurs over a small increase in plume buoyancy at the source particularly in power law mantle small buoyancy flux through the conduit affects only slow plumes whereas high flux causes plumes of different initial radius to reach the lithosphere with similar radius and excess temperature the results suggest that the study of important starting plumes those that will have considerable geological consequences can be simplified to that of 1 nonentraining constant radius thermals plume heads isolated from the source for small buoyancy flux and 2 nonentraining plumes for high flux in all cases most entrainment occurs in the lower mantle comparison between power law and newtonian mantle viscosities suggests that the upper mantle will respond as a power law fluid because it yields lower effective viscosity when submitted to the buoyancy stress of plumes than does a corresponding newtonian mantle the power law response of the upper mantle is unlikely to considerably influence ascent time of plumes but it will considerably change the way plumes interact with the lithosphere

impact of the lithosphere on dynamic topography insights from analogue modeling density anomalies beneath the lithosphere are expected to generate dynamic topography at the earth s surface due to the induced mantle flow stresses which scale linearly with density anomalies while the viscosity of the upper mantle is expected to control uplift rates however limited attention has been given to the role of the lithosphere here we present results from analogue modeling of the interactions between a density anomaly rising in the mantle and the lithosphere in a newtonian system we find that for instabilities with wavelengths of the same order of magnitude as lithosphere thickness the uplift rate and the geometry of the surface bulge are inversely correlated to the lithosphere thickness we also show that a layered lithosphere may modulate the topographic signal with respect to previous approaches our models represent a novel attempt to unravel the way normal stresses generated by mantle flow are transmitted through a rheologically stratified lithosphere and the resulting topographic signal

did rifting on australia s southern margin result from tectonic uplift subsidence analyses from australia s southern margin are used to investigate the idea that rifting on the margin was caused by an extensional lithospheric stress field induced by a hot thermal plume impinging on the base of the lithosphere from beneath the continental lithosphere is modelled as a thin sheet with depth dependent rheology the model does not explicitly include elastic flexural or thermal stresses the excess buoyancy of a thermal plume pushes the lithosphere upward by as much as 1 km thereby increasing its potential energy if the horizontal components of lithospheric stress are fixed at some distant plate boundary the lithosphere is a stress guide the lithospheric stress field above the plume will produce horizontal extension that may ultimately be followed by continental separation and seafloor spreading the uplift is modelled by a time dependent function which increases smoothly from zero to its peak value and then decreases back to zero using a plausible mantle rheology the amount of extension inferred at well sites on the margin typically 20 to 30 extension is explained by uplift of the lithosphere to approximately 900 m with a subsequent decay of the uplift over a period of approximately 60 m y the rise time of the uplift function is poorly constrained in the uplift model extension occurs in the subaerial environment during the late jurassic early cretaceous the initial stage of rapid subsidence observed at most well sites on the margin is due in large part to decrease of the vertical stress anomaly caused by the decay of the thermal plume or by the lithosphere moving away from above it the rapid subsidence is followed by very gradual continued subsidence attributed to cooling of the extended lithosphere as in previous studies

residual geoid anomalies in atlantic ocean basin relationship to mantle plumes the relations of geoid data from radar altimeter measurements geos 3 to water depth and geochemical variations along the mid atlantic ridge are investigated eight geophysical lines across the ridge in the atlantic ocean were constructed approximately normal to the local ridge trend slopes of geoid per kilometer of topographic relief show an asymmetry between the two sides of the ridge that generally is not matched by topography which has better symmetry between the two flanks subtracting a regional field based on previous work produces residual geoid anomalies whose source must be in the earth s outer 600 km the residual geoid anomalies of the north atlantic show striking correlation with the axis of the mid atlantic ridge but those in the south atlantic do not instead major positive residual geoid anomalies appear over the rio grande and walvis ridges determination of slope s residual geoid per million years crustal age reveals that s increases with greater residual geoid anomaly and with higher shallower crestal ridge topography the slope of 22 cm m y west of the azores high near 38 n suggests that the anomalously high crestal residual geoid and depth anomalies may not be entirely of thermal origin which may normally produce slopes only of about 8 cm m y as found near 28 n away from shallow ridge topography to account for this difference in slopes simply by a variation in mantle temperature would require a difference of about 940 c between the two sites which appears excessive also the decrease in topographic height with crustal age away from the azores high is much greater than predicted by theoretical cooling curves correlation of residual geoid anomalies with depth geochemical variations along the length of the mid atlantic ridge and the chemistry of the rio grande rise and walvis ridge suggest that many regions of anomalous residual geoid and crestal depth values may be in large part a function of crustal thickness and density variations induced by plume intrusion and volcanism in addition to their transient thermal effects

chronology of the palmer deep site antarctic peninsula a holocene palaeoenvironmental reference for the circum antarctic palmer deep sediment cores are used to produce the first high resolution continuous late pleistocene to holocene time series from the antarctic marine system the sedimentary record is dated using accelerator mass spectrometer radiocarbon methods on acid insoluble organic matter and foraminiferal calcite fifty four radiocarbon analyses are utilized in the dating which provides a calibrated timescale back to 13 ka bp reliability of resultant ages on organic matter is assured because duplicates produce a standard deviation from the surface age of less than laboratory error i e 50 years in addition surface organic matter ages at the site are in excellent agreement with living calcite ages at the accepted reservoir age of 1260 years for the antarctic peninsula spectral analyses of the magnetic susceptibility record against the age model reveal unusually strong periodicity in the 400 200 and 50 70 year frequency bands similar to other high resolution records from the holocene but so far unique f

the interaction of plume heads with compositional discontinuities in the earth s mantle the effects of compositional discontinuities of density and viscosity in the earth s mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique three specific problems are considered 1 a plume head rising away from a deformable interface 2 a plume head passing through an interface and 3 a plume head approaching the surface of the earth for the case of a plume attached to a free surface the calculated time dependent plume shapes are compared with experimental results two principal modes of plume head deformation are observed plume head elongation or the formation of a cavity inside the plume head the inferred structure of mantle plumes namely a large plume head with a long tail is characteristic of plumes attached to their source region and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high to low viscosity region as a rising plume head approaches the upper mantle most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle if the plume head moves from a high to low viscosity region the plume head becomes significantly elongated and for the viscosity contrasts thought to exist in the earth could extend from the 670 km discontinuity to the surface plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry the dynamic surface topography induced by plume heads is bell shaped when the top of the plume head is at depths greater than about 0 1 plume head radii as the plume head approaches the surface and spreads the dynamic topography becomes plateau shaped the largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical and the surface expression of these stresses is likely to be dominated by radial extension as the plume spreads compressional stresses on the surface are produced beyond the edges of the plume consequently extensional features will be produced above the plume head and may be surrounded by a ring of compressional features

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

onset of mantle plumes in the presence of preexisting convection the onset of plumes where broad scale convection already exists is investigated along with the thermal history of the earth geological evidence relating to the onset time of mantle plumes is not definitive no well defined hotspot tracks older than mesozoic have been identified but it is difficult to tell whether this absence is a reliable indication of a late onset of mantle plumes since only continental crust of this age exists using the hotspot track distribution for the pacific the absence of pre mesozoic hotspot tracks on the canadian shield cannot easily be attributed to chance however young continental hotspot tracks are controversial and it is not clear whether tracks such as the monteregian hills white mountain magma series would be recognized as such without considering the contributing evidence of the adjacent oceanic crust and its well defined seamount chains our simple models yield potentially testable predictions the instability of plume formation in a system where broad scale convection already exists depends on horizontal conduit flow in the basal boundary layer toward the plume source geographically plumes are most likely to form where the boundary layer is thickest and where broad scale ascending flow already exists parameterized convection indicates that the heat transport by conduit flow in the boundary layer toward plumes depends linearly on the ratio of effective viscosity for broad scale convection to effective boundary layer viscosity in terms of material properties this ratio is the ratio of viscosity across the boundary layer to about the 0 45 power for the case where the broad scale convection is heated from below the viscosity contrast across the boundary layer depends on the temperature contrast across the layer a late onset of mantle plumes is implied if the temperature contrast at the base of the mantle has increased with time that is if the core has cooled more slowly than the interior of the mantel in one scenario before about 3 b y b p the mantle temperature varied slowly because surface heat flow was nearly balanced by internal radioactivity during this time the mantle was hotter than present and its internal viscosity lower hie temperature contrast at the base of the mantel was thus small after 3 b y internal radioactivity no longer balanced surface heat flow and the mantle cooled about a few hundred degrees celsius as indicated by petrological observations the core cooled slower than the mantle and the temperature contrast across the bottom boundary layer increased with time leading to a late onset of plumes phenomenological inferences are obtained as venus may have cooled more rapidly than the earth its mantle may be too cool for rapid plate tectonics and the gravity field and the convective pattern are dominated by hot upwelling regions

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis