precambrian mafic dyke swarms in the singhbhum craton eastern india and their links with dyke swarms of the eastern dharwar craton southern india discussion

scratching the surface estimating dimensions of large igneous provinces a study of five major basaltic provinces including oceanic plateaus volcanic passive margins and continental flood basalts shows that they are voluminous constructions of extrusive igneous rock underlain by intrusive rock crustal thickness ranges from 20 to 40 km and lower crust is characterized by high 7 0 7 6 km s seismic velocities volumes and emplacement rates derived for two oceanic plateaus the ontong java and kerguelen broken ridge reveal short lived pulses of increased global crustal production and suggest an origin involving the lower mantle the ontong java rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid ocean ridge system despite the importance off large igneous provinces in studies of mantle dynamics and the global environment scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features

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

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

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

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

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

direct thermoluminescence chronology for early iron age smelting technology on the gambaga escarpment ghana abstract we present luminescence dates which demonstrate a full blown smelting technology in northern ghana west africa during the early iron age our chronology is based on thermoluminescence tl dating of quartz grains extracted from the walls of three iron smelters located at the birimi site in the northern region of ghana two of the smelters yielded statistically indistinguishable ages of 1080 70 and 1090 60 years while the third yielded a higher age of 1600 100 years all are significantly older than the sole direct radiocarbon date of 550 330 calbp 460 90 bp obtained on a furnace in the northern region the tl ages indicate that iron smelting was well established in the northern region before the middle of the first millennium ad and corroborate the validity of a number of similarly early radiocarbon dates associated with early iron age ceramics but not directly associated with smelting activity from other sites in northern ghana the iron working remains at birimi are located on a river terrace only a few tens of meters west of a dense kintampo occupation radiocarbon and tl dates for the kintampo complex demonstrate that the iron age and kintampo components at birimi are clearly non contemporaneous the occurrence of two distinct layers of goethite within 150 cm of the terrace surface both of which are clearly exposed in the scarp of the terrace upon which the smelters are situated indicates that the industrial site was deliberately sited to take advantage of the readily available source material this premise is confirmed by geochemical analyses of sediment goethite and slag from the smelting site we therefore propose that the frequent proximity of kintampo and iron production sites is incidental in the sense that each group exploited different resources offered by a riverbank location

the timing of crustal extension and the eruption of continental flood basalts the association between mantle plumes continental flood basalt eruption and the extension and rifting of the crust is well recognized1 3 but there has always been controversy as to whether the plume or the rifting was the initiating factor4 5 was rifting triggered by doming above the thermal plume2 3 6 or was rifting the driving force preceding the associated volcanicity7 8 here i show that recent flow by flow mapping of two continental flood basalt provinces the columbia river basalt group and the deccan basalt group allows one to infer the relative timing of volcanicity and crustal extension in both provinces eruption of the main tholeiitic phase preceded significant extension and crustal thinning this suggests that the primary event in the formation of these provinces was the upwelling of a mantle plume and that the subsequent crustal extension rifting and decompression was in part a consequence of the initiation of that plume

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

granular avalanches on the moon mass wasting conditions processes and features seven lunar crater sites of granular avalanches are studied utilizing high resolution images 0 42 1 3 m pixel from the lunar reconnaissance orbiter camera one in kepler crater is examined in detail all the sites are slopes of debris extensively aggraded by frictional freezing at their dynamic angle of repose four in craters formed in basaltic mare and three in the anorthositic highlands diverse styles of mass wasting occur and three types of dry debris flow deposit are recognized 1 multiple channel and lobe type with coarse grained levees and lobate terminations that impound finer debris 2 single surge polylobate type with sub parallel arrays of lobes and fingers with segregated coarse grained margins and 3 multiple ribbon type with tracks reflecting reworked substrate minor levees and no coarse terminations the latter type results from propagation of granular erosion deposition waves down slopes dominantly of fine regolith and it is the first recognized natural example dimensions architectures and granular segregation styles of the two coarse grained deposit types are like those formed in natural and experimental avalanches on earth although the timescale of motion differs due to the reduced gravity influences of reduced gravity and fine grained regolith on dynamics of granular flow and deposition appear slight but we distinguish for the first time extensive remobilization of coarse talus by inundation with finer debris the few sites show no clear difference attributable to the contrasting mare basalt and highland megaregolith host rocks and their fragmentation this lunar study offers a benchmarking of deposit types that can be attributed to formation without influence of liquid or gas

routing debris flows with particle segregation a computer simulation method is developed to predict the temporal variations of debris flow discharge solids concentration and vertical mean particle size distribution a single finite step in the simulation is divided into two stages in the first stage a sediment bed composed of a well graded mixture is eroded by surface water introduced at the head of a channel thereby generating a debris flow in the second stage larger particles in the debris flow move upward because of dispersive force and are then transported forward faster than smaller particles which remain in the lower part of the flow because the velocity is slower in the lower layer another numerical simulation method is also developed to predict simultaneously the shape of the debris deposit area the distribution of the deposit thicknesses within the area and the three dimensional distribution of the mean diameters of the deposited particles both simulation methods are verified using laboratory experiments

fine grained linings of leveed channels facilitate runout of granular flows abstract catastrophic dense granular flows such as occur in rock avalanches debris flows and pyroclastic flows move as fully shearing mixtures that have approximately 60 vol solids and tend to segregate to form coarse grained fronts and leveed channels levees restrict spreading of unconfined flows and form as coarse particles that become concentrated in the top of the flow are transported to the front and then advect to the sides in the flow head channels from which most material has drained away down slope are commonly lined with fine grained deposit widely thought to remain from the tail of the waning flow we show how segregation in experimental dense flows of carborundum or sand 300 425 m mixed with spherical fine ballotini 150 250 m on rough slopes of 27 29 produces fine grained channel linings that are deposited with the levees into which they grade laterally maximum runout distance is attained with mixtures containing 30 40 sand just sufficient to segregate and form levees that are adequately robust to restrict the spreading attributable to the low friction fines resin impregnation and serial sectioning of deliberately arrested experimental flows shows how fines lined levees form from the flow head the flows create their own stable conduit entirely from the front which in a geophysical context can play an important mechanistic role in facilitating runout the flow self organization ensures that low friction fines at the base of the segregated channel flow shear over fine grained substrate in the channel thus reducing frictional energy losses we propose that in pyroclastic flows and debris flows which have considerable mobility attributable to pore fluid pressures such fine grained flow contact zones form similarly and not only reduce frictional energy losses but also reduce flow substrate permeability so as to enhance pore fluid pressure retention thus the granular flow self organization that produces fine grained channel linings can be an important factor in facilitating long runout of catastrophic geophysical flows on the low slopes few degrees of depositional fans and aprons around mountains and volcanoes

steady inclined flows of granular fluid mixtures we extend a recent theory for steady uniform gravity driven flow of a highly concentrated granular fluid mixture over an erodible bed between frictional sidewalls we first include angles of inclination greater than the angle of repose of the particles then we introduce a boundary condition for flow over a rigid bumpy bed we compare the predictions of the resulting theory with the volume flow rates depths and angles of inclination measured in the experiments on dry and variously saturated flows over rigid and erodible boundaries finally we employ the resulting theory with the assumption that the flow is shallow to solve in an approximate way for the variation of height and average velocities along a steady non uniform inclined flow of a granular fluid mixture that moves over a rigid bumpy bed the solutions exhibit features of the flow seen in the experiments for example a dry bulbous snout in advance of the fluid whose length increases with increasing number of the particles and that disappears with increasing velocity for which satisfactory explanations were lacking

shock waves dead zones and particle free regions in rapid granular free surface flows shock waves dead zones and particle free regions form when a thin surface avalanche of granular material flows around an obstacle or over a change in the bed topography understanding and modelling these flows is of considerable practical interest for industrial processes as well as for the design of defences to protect buildings structures and people from snow avalanches debris flows and rockfalls these flow phenomena also yield useful constitutive information that can be used to improve existing avalanche models in this paper a simple hydraulic theory first suggested in the russian literature is generalized to model quasi two dimensional flows around obstacles exact and numerical solutions are then compared with laboratory experiments these indicate that the theory is adequate to quantitatively describe the formation of normal shocks oblique shocks dead zones and granular vacua such features are generated by the flow around a pyramidal obstacle which is typical of some of the defensive structures in use today

effects of debris flow composition on runout depositional mechanisms and deposit morphology in laboratory experiments predicting debris flow runout is of major importance for hazard mitigation apart from topography and volume runout distance and area depends on debris flow composition and rheology but how is poorly understood we experimentally investigated effects of composition on debris flow runout depositional mechanisms and deposit geometry the small scale experimental debris flows were largely similar to natural debris flows in terms of flow behavior deposit morphology grain size sorting channel width depth ratio and runout deposit geometry lobe thickness and width in our experimental debris flows is largely determined by composition while the effects of initial conditions of topography i e outflow plain slope and channel slope and width and volume are negligible we find a clear optimum in the relations of runout with coarse material fraction and clay fraction increasing coarse material concentration leads to larger runout however excess coarse material results in a large accumulation of coarse debris at the flow front and enhances diffusivity increasing frontal friction and decreasing runout increasing clay content initially enhances runout but too much clay leads to very viscous flows reducing runout runout increases with channel slope and width outflow plain slope debris flow volume and water fraction these results imply that debris flow runout depends at least as much on composition as on topography this study improves understanding of the effects of debris flow composition on runout and may aid future debris flow hazard assessments

the perfect debris flow aggregated results from 28 large scale experiments 1 aggregation of data collected in 28 controlled experiments reveals reproducible debris flow behavior that provides a clear target for model tests in each experiment 10 m3 of unsorted water saturated sediment composed mostly of sand and gravel discharged from behind a gate descended a steep 95 m flume and formed a deposit on a nearly horizontal runout surface experiment subsets were distinguished by differing basal boundary conditions 1 versus 16 mm roughness heights and sediment mud contents 1 versus 7 percent dry weight sensor measurements of evolving flow thicknesses basal normal stresses and basal pore fluid pressures demonstrate that debris flows in all subsets developed dilated coarse grained high friction snouts followed by bodies of nearly liquefied finer grained debris mud enhanced flow mobility by maintaining high pore pressures in flow bodies and bed roughness reduced flow speeds but not distances of flow runout roughness had these effects because it promoted debris agitation and grain size segregation and thereby aided growth of lateral levees that channelized flow grain size segregation also contributed to development of ubiquitous roll waves which had diverse amplitudes exhibiting fractal number size distributions despite the influence of these waves and other sources of dispersion the aggregated data have well defined patterns that help constrain individual terms in a depth averaged debris flow model the patterns imply that local flow resistance evolved together with global flow dynamics contradicting the hypothesis that any consistent rheology applied we infer that new evolution equations not new rheologies are needed to explain how characteristic debris flow behavior emerges from the interactions of debris constituents

isotopic evidence for the origin of the manihiki and ontong java oceanic plateaus abstract pb nd and sr isotopic results for lavas of the cretaceous ontong java and manihiki oceanic plateaus fall well within the modern day oceanic island or hotspot field the data provide no evidence of old continental basements but indicate a major involvement of kerguelen type or em i like mantle in the sources of both plateaus which appear to have probably been formed at least in part by hotspots however the presently active hotspots that pacific plate reconstructions suggest might have been possible plateau sources lack kerguelen type isotopic compositions either these hotspots did not participate in the formation of the two plateaus or if they did kerguelen type material must have been volumetrically much more important early in their existence two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical geochronological and geophysical data the first holds that the plateaus formed cataclysmically in association with surfacing plume heads the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation a near ridge origin appears to be indicated by evidence that most of the pacific plateaus were built largely on relatively young ocean crust however we suggest that a near ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation which should be substantially greater than for ordinary hotspots in either case the lack of hotspot tracks seamount chains attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near ridge setting

tectonic controls on magmatism associated with continental break up an example from the parana etendeka province the high and low ti basalts of the parana etendeka province were primarily derived from old trace element enriched source regions in the lithospheric mantle and they are associated with dyke swarms of different orientations these swarms appear to reflect different amounts of extension and it is inferred that the high and low ti magma types were characterised by different melt generation rates of 0 15 km3 yr 1 and 0 4 km3 yr 1 respectively stewart et al earth planet sci lett 143 1996 95 109 there is probably a gap of 2 myr between the end of the main phase of cfb magmatism and the oldest rocks on the adjacent ocean floor a simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous 0 2 h2o peridotite solidus in the lithosphere for a maximum of 4 and a duration of extension of 10 myr the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature tp and is relatively insensitive to the thickness of the mbl while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting for a tp of 1450 50 c inferred from the crustal thickness estimates along the rio grande rise and walvis ridge the model successfully generates 2 4 km of lithosphere derived melt before producing significant volumes of asthenosphere derived melt it is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere critically in areas of significant melt generation within the mantle lithosphere during extension and break up there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere no such gap is present if all melts are generated within the mantle plume and thus these models may in principle be tested in the geologic record

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

late pleistocene and holocene sea level change in the australian region and mantle rheology summary spatial and temporal variations in sea level are produced by the melting of the late pleistocene ice and by the earth s response to the redistribution in surface loads by examining different parts of the sea level curves of the past 20 000 yr from geographically widely distributed regions it becomes possible to constrain models of the melting history of the ice sheets and of the earth s rheology observations from sites away from the former arctic ice sheets such as the australian and south pacific region are particularly important for constraining the total meltwater volumes added into the oceans in the past 20 000 yr and the rates at which this occurred these observations indicate that the antarctic ice sheets provided a significant contribution to the sea level rise at a rate that was approximately synchronous with the melting of the laurentide ice sheet except for the interval 9000 6000 yr ago when it may have lagged behind minor melting of the antarctic ice sheet appears to have continued throughout the late holocene differential observations of the late holocene sea level change recorded at sites in the same region are particularly useful for estimating parameters describing the earth s non elastic response to surface loading the effective parameters used here are a lithospheric thickness an upper mantle viscosity and a lower mantle viscosity describing the response below 670 km depth with the observations used here it is not possible to separate the lithospheric thickness h from the upper mantle viscosity and the viscosity results are based on the assumption that 50 h 100 km neither do these observations provide a resolution of the depth dependence of viscosity in the upper mantle and the resulting estimates are effective parameters only differential sea levels along continental margins and along the shores of large gulfs and bays constrain the effective upper mantle viscosity to be about 1 2 1020 pa s 1 while differential values from islands of different sizes are suggestive of a somewhat lower value the lower mantle taken to be below 670 km depth viscosity is about two orders of magnitude greater than this the estimated ice and rheological models explain many of the holocene sea level observations throughout the australian and southern pacific region the tilting of continental margins as exemplified by observations of variable amplitudes of the holocene high stands and the variable times at which sea levels first reached their present level along the north queensland coast and great barrier reef is well represented by these models differential holocene sea levels observed along the narrow spencers gulf of south australia are also well explained by the models and in neither case is it necessary to invoke tectonic motions predicted late holocene sea levels at small to medium sized islands are characterized by small amplitude high stands that reached their maximum values about 4000 2000 yr ago consistent with observations from the society cook and tuamotu islands

large scale landslide simulations global deformation velocities and basal friction the cause of the apparent small friction exhibited by long runout landslides has long been speculated upon in an attempt to provide some insight into the matter this paper describes results obtained from a discrete particle computer simulation of landslides composed of up to 1 000 000 two dimensional discs while simplified the results show many of the characteristics of field data the volumetric effect on runout preserved strata etc and with allowances made for the two dimensional nature of the simulation the runouts compare well with those of actual landslides the results challenge the current view that landslides travel as a nearly solid block riding atop a low friction basal layer instead they show that the mass is completely shearing and indicate that the apparent friction coefficient is an increasing function of shear rate the volumetric effect can then be understood with all other conditions being equal different size slides appear to travel with nearly the same average velocity however as the larger landslides are thicker they experience smaller shear rates and correspondingly smaller fractional resistance

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

flood basalts and ocean island basalts a deep source or shallow entrainment abstract basalts from continental flood basalts cfbs and intraplate or hotspot ocean islands are found to have distinct geochemical signatures this diversity in composition is generally believed to result from the upwelling plume entraining overlying reservoirs of shallow and intermediate depth mantle material during its ascent from the deep mantle here we present laboratory experiments and numerical model calculations which clarify that for a strongly temperature dependent viscosity like that of the mantle a rising plume head should be expected to bring up a surrounding sheath of deep mantle from its source region mixing between the central core of the plume and this sheath produces the whorl like structures noted in previous studies where they were typically attributed to thermal entrainment of surrounding ambient mantle but this is mainly the product of intermixing between neighboring parts of the plume s deep source material these results imply that the popular idea that mantle plumes should typically mix small fractions of deep primitive mantle material with much larger fractions of shallower depleted mantle needs to be critically reexamined

bulbous head formation in bidisperse shallow granular flow over an inclined plane rapid shallow granular flows over inclined planes are often seen in nature in the form of avalanches landslides and pyroclastic flows in these situations the flow develops an inversely graded large at the top particle size distribution perpendicular to the plane as the surface velocity of such flows is larger than the mean velocity the larger material is transported to the flow front this causes size segregation in the downstream direction resulting in a flow front composed of large particles since the large particles are often more frictional than the small the mobility of the flow front is reduced resulting in a so called bulbous head this study focuses on the formation and evolution of this bulbous head which we show to emerge in both a depth averaged continuum framework and discrete particle simulations furthermore our numerical solutions of the continuum model converge to a travelling wave solution which allows for a very efficient computation of the long time behaviour of the flow we use small scale periodic discrete particle simulations to calibrate close our continuum framework and validate the simple one dimensional 1 d model with full scale 3 d discrete particle simulations the comparison shows that there are conditions under which the model works surprisingly well given the strong approximations made for example instantaneous vertical segregation

nature of continental crust along the narmada son lineament inferred from gravity and deep seismic sounding data abstract the narmada son lineament nsl is a very prominent feature on the tectonic map of india and divides the indian peninsula into two parts the northern part characterised by lower elevations and more positive bouguer gravity anomalies as compared to the southern part the lineament is characterised by a gravity high practically all along its length four published deep seismic sounding dss profiles across the nsl give vital information concerning its origin an analysis of the bouguer gravity field along these profiles has been carried out using all available geological and velocity density information the analysis reveals that the areas immediately south of the nsl are characterised by higher density crust in comparison to the areas north of it this increase in density could have taken place during mesozoic to tertiary times when major tectonic volcanic activity took place over peninsular india the presence of deep seated faults sediments and anomalous crust shows that the lineament has some characteristics of a rift valley

grain size segregation and levee formation in geophysical mass flows 1 data from large scale debris flow experiments are combined with modeling of particle size segregation to explain the formation of lateral levees enriched in coarse grains the experimental flows consisted of 10 m3 of water saturated sand and gravel which traveled 80 m down a steeply inclined flume before forming an elongated leveed deposit 10 m long on a nearly horizontal runout surface we measured the surface velocity field and observed the sequence of deposition by seeding tracers onto the flow surface and tracking them in video footage levees formed by progressive downslope accretion approximately 3 5 m behind the flow front which advanced steadily at 2 m s 1 during most of the runout segregation was measured by placing 600 coarse tracer pebbles on the bed which when entrained into the flow segregated upwards at 6 7 5 cm s 1 when excavated from the deposit these were distributed in a horseshoe shaped pattern that became increasingly elevated closer to the deposit termination although there was clear evidence for inverse grading during the flow transect sampling revealed that the resulting leveed deposit was strongly graded laterally with only weak vertical grading we construct an empirical three dimensional velocity field resembling the experimental observations and use this with a particle size segregation model to predict the segregation and transport of material through the flow we infer that coarse material segregates to the flow surface and is transported to the flow front by shear within the flow head coarse material is overridden then recirculates in spiral trajectories due to size segregation before being advected to the flow edges and deposited to form coarse particle enriched levees

geothermal constraints on emeishan mantle plume magmatism paleotemperature reconstruction of the sichuan basin sw china the middle late permian emeishan large igneous province elip in southwestern china represents a classic example of a mantle plume origin to constrain the thermal regime of the elip and contemporaneous magmatic activity in the northeastern sichuan basin maximum paleotemperature profiles of deep boreholes were reconstructed using vitrinite reflectance ro and apatite fission track data two heating patterns were identified 1 heating of the overlying lithosphere by magma storage regions and or magmatic activity related to the mantle plume which resulted in a relatively strong geothermal field and 2 direct heating of country rock by stock or basalt borehole ro data and reconstructed maximum paleotemperature profiles near the elip exhibit abrupt tectonothermal unconformities between the middle and late permian the profiles in the lower subsections i e pre middle permian exhibited significantly higher gradients than those in the upper subsections distal to the basalt province high paleo geotemperatures hereafter paleotemperatures were inferred despite deformation of the paleogeothermal curve due to deep faults and igneous rocks within the boreholes in contrast ro profiles from boreholes without igneous rocks i e late permian contained no break at the unconformity paleotemperature gradients of the upper and the lower subsections and erosion at the middle late permian unconformity revealed variations in the thermal regime the inferred spatial distribution of the paleothermal regime and the erosion magnitudes record the magmatic and tectonic thermal response to the emeishan mantle plume

mantle plume heads and the initiation of plate tectonic reorganizations we present results of numerical simulations which examine the plausibility of a reorganization of plate motions brought about by the interaction of a hot low viscosity plume head with the underside of a tectonic plate a numerical model of highly viscous fluid flow driven by thermal buoyancy is employed to examine the interaction of mantle plumes with surface plates in a two dimensional cartesian geometry plate like boundary conditions applied to the upper surface consist of a large active plate with fixed speed plus a smaller passive plate whose speed and direction of motion are permitted to react to tractions induced by the underlying mantle flow driven by the active plate the passive plate is also subject to applied end loads intended to simulate incipient subduction the mantle plume is initiated with a hot patch on the lower boundary and viscosity varies with temperature following an arrhenius rheology the calculations performed with fixed aspect ratio 4 1 and fixed rayleigh number ra 106 where ra refers to the strength of the starting plume were found to depend on three parameters the strength of the viscosity variation the size of the applied end load and the speed of the active plate these calculations indicate that plate motions can be sharply influenced by the lubricating effects of mantle plume heads even to the point of causing a reversal in the direction of plate motion

magnetic anomaly lineations from late jurassic to early cretaceous in the west central pacific ocean summary late jurassic to early cretaceous mesozoic magnetic anomaly lineations the phoenix magellan mid pacific mountains hawaiian and japanese lineation sets with fracture zones in the west central pacific ocean were identified more comprehensively than in any previous studies we fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks two remarkable fracture zones the phoenix and central pacific fracture zones were mapped and newly named our newly identified lineations from mlon to mo around the mid pacific mountains which belong to the hawaiian lineation set illustrated that the sea floor south of the mid pacific mountains has the same age as that of the north 132 118 ma our analysis of skewness parameters revealed that the older part of the phoenix set m17 m29 has skewness different from that of the younger part ml m14 implying an effect of magnetic overprints by the cretaceous volcanism it was confirmed that the spreading rate of the mesozoic pacific spreading system was the fastest in the world in the mesozoic a drastic change in spreading rates occurred simultaneously at the period between chrons m21 and m20 149 5 148 5 ma in all the mesozoic pacific spreading systems the event appears to be synchronous with events in other oceans such as the mesozoic atlantic and indian oceans