structural characterization of natural loess and remolded loess under triaxial tests this paper characterizes the microstructural evolution of saturated natural loess from a site in jingyang china during triaxial test along two stress paths for comparison the same soil in remolded state is also characterized mercury intrusion porosimetry mip scanning electron microscopy sem and environmental scanning electron microscopy esem were used for the microstructure investigation results show that the microstructure of natural loess is similar to that of remolded loess before the consolidated undrained cu triaxial tests including conventional triaxial compression ctc test and reduced triaxial compression rtc test on the contrary their microstructures are different after the tests deeper examination shows that loading led to a significant change of inter aggregate pores without modifying the intra aggregate pores in addition the volume of inter aggregate pores after the rtc test is greater than that after the ctc test under the same consolidation stress the microstructure features of natural and remolded loess after the tests are further used to analyze the macro mechanical behavior for better understanding the relationship between them the results show that it is the cementation bonds of which breakage is influenced by stress path and confining pressure that play important roles in the strength and deformation behavior

a new method for evaluating loess collapsibility and its application abstract collapsibility affects loess engineering stability currently there are two methods used to determine the collapsibility of loess the traditional indoor collapsibility test indoor test method and the field immersion test field test method however for the same loess site the collapse values under overburden pressure obtained by the two methods are quite different for sites that cannot be tested in situ it is easy to misjudge the collapsibility of the loess foundation when depending only on indoor test results to improve the accuracy of the indoor collapsibility determination the characteristics of the compression curves obtained from confined compression tests at six different loess sites are analyzed in this paper the compression curves of loess under different states are normalized and described by the composite power exponential model the collapsibility of loess is closely related to its structure a new method for analyzing and evaluating the collapsibility of loess is proposed the method combines the relationship between the compression characteristics and initial structural properties of loess and the relationship between the initial structural properties and physical comprehensive indexes to verify the applicability and accuracy of the new method the collapse values under overburden pressure of two test sites along the xi an metro line 5 were calculated using the physical indexes of loess at different depths and compared with the values calculated from the indoor test and field measured values the results show that the calculated value of the new method is closer to the measured value of the field test than is the calculated value of the indoor test and the new method can correctly determine the collapsibility type and grade of the site the new method provides a simple and feasible means by which to correctly and quickly evaluate site collapsibility in loess engineering which is beneficial for conducting loess site collapsibility evaluations under full operation to develop and implement the measures of foundation treatment in loess areas the method can be used as a reference at loess sites where field tests cannot be performed

a high resolution p wave model for the top 1200 km of the mantle this work attempts to map aspherical velocities in the top 1200 km of the mantle that are illuminated by the international seismological centre p wave data the processing includes summary ray sorting and hypocentral redetermination to cope with the extreme variation of ray coverage a multicell inversion is used to simultaneously constrain anomalies in overlapping cells the multicell solutions are summed after inversion to form the final model named p1200 at cell size of 1 1 and about 50 km in thickness the lateral resolution of the model is about 1 2 in major subduction zones and about 5 15 in most places major features in the model withstood tests on signal coherency and different reference models the long wavelength parts of the p1200 model correlate well with previous global models especially the s12 model of shear wave among the large features in the transition zone and below there are two fast bands one from canada to south america and another from siberia through east asia and the philippine sea to western australia there are also slow patches under the central pacific and from india to eastern africa though most subduction zones have slab like high velocity anomalies they are sandwiched by broad slow velocities at shallow depths some of the slabs appear to penetrate the 660 km discontinuity and stagnant slabs are also seen in the transition zone plume like slow anomalies exist below some prominent hot spots like hawaii iceland yellowstone and french polynesia

widespread palaeocene volcanism around the northern north atlantic and labrador sea evidence for a large hot early plume head abstract voluminous volcanism took place west of greenland and northwest of the british isles in the palaeocene 6 7 ma before the start of sea floor spreading in the northern north atlantic very hot picritic magmas were produced simultaneously in such widely separated areas that they could not all have been above the central stem of the same mantle plume a single large plume head with temperatures of c 1500 c over a diameter of c 2000 km can explain the observations if surface volcanic activity took place only where the lithosphere was thin the presence of picrites does not indicate proximity to the plume stem

an admittance study of the reykjanes ridge and elevated plateaux between the charlie gibbs and senja fracture zones summary the north atlantic between the charlie gibbs and senja fracture zones shows a number of oceanic plateaux continental fragments and unusual mid oceanic ridges the reykjanes ridges and the iceland plateau being the most prominent ones while rifted continental fragments and marginal volcanic plateaux have been formed during the initial formation process of the north atlantic others such as the reykjanes ridge and plume related plateaux such as iceland and possibly also jan mayen are still under development by studying the ratio of geoid to topography in the long wavelength range for these regions of elevated topography and comparing the results with theoretical models of crustal loading and subsurface thermal uplift we can show that the rockall bank and the voring plateau are compensated by a thick crust alone while the iceland plateau the outer faeroe plateau and the jan mayen block have a strong component of deeply situated mass anomalies the admittance the spectral ratio of geoid to topography for a number of profiles across the reykjanes ridge clearly indicates a decreas ei n compensation depth with distance from iceland this strengthens the argument for a mass flux from the iceland plume into the reykjanes ridge

viscous heating in silicate melts an experimental and numerical comparison 1 the transition from newtonian to non newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate shear thinning may strongly influence the dynamics of magmatic flows but the details of its microscopic origins are not fully understood here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning we compare the results of laboratory uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup both the experimental and numerical results concord and indicate that the reduction of the temperature dependent viscosity in flowing silicate melts is a result of viscous heating viscous heating was quantified for glasses with viscosities ranging from 108 to 1011 pa s and strain rates from 10 5 to 100 s 1 the results of 48 compression experiments indicate that the transition from newtonian to non newtonian flow in the silicate melt occurs at a brinkmann number i e ratio of heat gained to heat lost around 1 whereas brittle behavior dominates the melt deformation when the deborah number i e ratio of viscoelastic relaxation time to characteristic deformation time is larger than around 0 01 the observed viscous heating significantly contributes to the viscosity decrease observed in high stress strain rate experiments and questions our current understanding of the non newtonian deformation behavior of silicate melts

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

field study on the reinforcement of collapsible loess using dynamic compaction abstract the dynamic compaction dc method is an effective ground treatment technique that is widely used for a variety of soil types and conditions particularly in loess areas however dc is rarely applied to ground where thick collapsible loess is present the conventional dc method exerts an energy level of no more than 5000 kn m although the dc for collapsible loess treatment is increasing the use of higher energy inputs for similar soil conditions is quite limited in this paper dynamic compaction with a maximum energy level of 12 000 kn m was applied to a collapsible loess site in the city of qingyang in the gansu province in northwestern china field tests were conducted to determine the optimum dc operational parameters the field studies included deformation tests standard penetration tests spt static penetration tests and plate load tests the deformation tests included the crater depth per drop and the whole test zone elevations before and after dc the standard and static penetration tests and plate load tests were performed to evaluate the final effect of dc the collapsibility was also measured before and after dc the allowable ground bearing capacity and the depth of improvement at the site after treatment achieved no less than 250 kpa and 10 m respectively and the collapsibility was greatly reduced or was completely eliminated

effects of soil structure on soil behaviour illustrated with loess glacially loaded clay and simulated flaser bedding examples soil structure is considered to be a systematic homogeneously repetitive heterogeneity of properties and conditions of soil for upscaling and homogenisation of properties and conditions it is necessary to understand the ways in which soil structures occur and to obtain understanding of the ways in which it influences soil behaviour using three case studies different aspects of the effects of soil structure in soil mechanics are discussed the very loose grain scale packing of loess deposits held together by weak structural bonds between particles results in dramatic propagating failure when the strength of the bonds is exceeded by deformations as illustrated in the first case nonhorizontal stresses induced in the subsurface below moving continental ice sheets result in rotated stress fields within the affected soil the rotated stress field is stored in the structure of the soil as is shown in the second case and influences present day properties and behaviour of these clays it is shown in the third case that there is a large difference in the behaviour of clayey sand depending on whether the clay occurs in the form of the common clay flasers or whether the clay is homogeneously dispersed in the sand the deformation and stress propagation in the case of sand with clay flasers results in local failures and shear planes due to the large contrast in the properties of the materials making up the sedimentary structure the result is that many of the properties of sand with flasers are rather more like the clay of the flasers than the sand incorporation in the engineering practice of effects of common types of soil structure is made possible with the widespread availability of techniques such as numerical modelling these techniques enable the development of appropriate tools for practical application

geological hazards in loess terrain with particular reference to the loess regions of china abstract the considerable morphodynamic energy provided by the continuing tectonic evolution of asia is expressed in high erosional potentials and very high rates of sediment production that make this continent unequalled as a terrestrial source of primary silt many of these environments are hazardous threatening human occupation health and livelihood especially in regions of dense population such as the loess lands of north china dry loess can sustain nearly vertical slopes being perennially under saturated however when locally saturated it disaggregates instantaneously such hydrocompaction is a key process in many slope failures made worse by an underlying mountainous terrain of low porosity rocks gully erosion of loess may yield very high sediment concentrations 60 by weight characteristic vertical jointing in loess influences the hydrology enlarged joints develop into natural sub surface piping systems which on collapse produce a loess karst terrain collapsible loess up to 20 m thick is common on the western loess plateau foundation collapse and cracked walls are common many rapid events following periods of unusually heavy monsoonal rain slope failure is a major engineering problem in thick loess terrain flow slide and spread types being common the results are often devastating in both urban and rural areas an associated hazard is the damming of streams by landslides the human population increases the landslide risk in china notably through imprudent land use practices including careless water management a number of environmentally related endemic diseases arise from the geochemistry of loess and its groundwaters including fluorosis cretinism kaschin beck disease keshan disease and goitre the chinese desert margins also have a major atmospheric dust problem the effect of such dust upon human health in these extensive regions including many large cities has yet to be evaluated but pneumoconiosis is thought to affect several million people in north and west china

steady plumes in viscously stratified vigorously convecting three dimensional numerical mantle convection models with mobile plates 1 we present results from three dimensional 3 d cartesian geometry computations featuring vigorous high rayleigh number mantle convection in a system incorporating stiff tectonic plates with dynamically determined time dependent velocities we track plume related hot spot motion while calculating associated synthetic hot spot tracks allowing us to estimate plume migration rates relative to plate motion rates plate like surface motion is achieved explicitly by specifying both the plate geometry and rigidity however plate velocities evolve dynamically in response to the buoyancy distribution within the convecting system including buoyancy within the plate itself we find that convection is characterized by downwelling sheets and varying numbers of 3 d upwelling structures when the lower mantle to upper mantle viscosity ratio is varied from 9 to 90 as the lower mantle viscosity is increased relative to the upper mantle viscosity the upwellings in the calculations evolve into vigorous active plumes characterized by long narrow thermal conduits with broad disk like heads the total number shape and persistence of the plumes are affected by the specified viscosity stratification in the calculations our findings indicate that hot spots associated with mantle plumes drift by smaller distances in comparable amounts of time when the lower mantle viscosity is increased relative to the upper mantle viscosity in cases with a high lower mantle to upper mantle viscosity ratio the depth of the flow aligned with plate motion is diminished and the plate scale return flow is integrated into a deep unorganized sluggish layer as a result plume conduit morphology in the lower mantle of such calculations is less influenced by plate motion than in calculations with a small difference between upper mantle and lower mantle viscosity for lower mantle viscosities 30 times and 90 times greater than the upper mantle viscosity the lower mantle velocity field is dominated locally by active plumes anchored in the lower thermal boundary layer of the convecting system and plume migration rates are typically only about 10 of the overriding plate velocity in such cases we also find that plume related hot spots persist for periods that we estimate to be between 0 55 and 1 1 plume transit times

use of multicomponent seismic data for oil water discrimination in fractured reservoirs geophysicists have devoted great efforts to the problem of determining fluid saturation from seismic measurements with some notable successes it is commonly believed that fluid information is to be found in the p wave data with shear waves being insensitive to fluid and indeed almost all successful fluid detection methodologies have been based on analysis of the p wave

effects of aligned fractures on the response of velocity and attenuation ratios to water saturation variation a laboratory study using synthetic sandstones p wave to s wave ratios are important seismic characterization attributes velocity ratios are sensitive to the petrophysical properties of rocks and to the presence of gas attenuation ratios have also been shown to be sensitive to the presence of partial liquid gas saturation the relationship between liquid gas saturation and p wave and s wave ratios has been used to distinguish gas saturated rocks from liquid saturated rocks aligned fractures are common in the earth s crust and cause seismic anisotropy and shear wave splitting however most existing relationships between partial gas liquid saturation and p wave and s wave ratios are for non fractured rocks we present experimental results comparing the effects of changing water saturation on qs qp versus vp vs ratios between a non fractured rock and one containing fractures aligned parallel to wave propagation direction we also study the effects of aligned fractures on the response of vp vs to changing water saturation using synthetic fractured sandstones with fractures aligned at 45o and parallel to the wave propagation direction the results suggest that aligned fractures could have significant effects on the observed trends some of which may not be obvious fractures aligned parallel to wave propagation could change the response of qs qp versus vp vs ratios to water saturation from previously reported trends shear wave splitting due to the presence of aligned fractures results in two velocity ratios vp vs1 and vp vs2 the fluid independence of shear wave splitting for fractures aligned parallel to wave propagation direction means the difference between vp vs1 and vp vs2 is independent of water saturation for fractures aligned at oblique angles shear wave splitting can be sensitive to water saturation and consequently be frequency dependent which can lead to fluid and frequency dependent differences between vp vs1 and vp vs2 the effect of aligned fractures on vp vs ratios not only depends on the fracture effects on both p wave and s wave velocities but also on the effects of water saturation distribution on the rock and fracture stiffness and hence on the p wave and s wave velocities as such these effects can be frequency dependent due to wave induced fluid flow a simple modelling study combining a frequency dependent fractured rock model and a frequency dependent partial saturation model was used to gain valuable interpretations of our experimental observations and possible implications which would be useful for field seismic data interpretation

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

the role of rheology and slab shape on rapid mantle flow three dimensional numerical models of the alaska slab edge 1 away from subduction zones the surface motion of oceanic plates is well correlated with mantle flow direction as inferred from seismic anisotropy however this correlation breaks down near subduction zones where shear wave splitting studies suggest the mantle flow direction is spatially variable and commonly non parallel to plate motions this implies local decoupling of mantle flow from surface plate motions yet the magnitude of this decoupling is poorly constrained we use 3d numerical models of the eastern alaska subduction transform plate boundary system to further explore this decoupling in terms of both direction and magnitude specifically we investigate the role of the slab geometry and rheology on the mantle flow velocity at a slab edge the subducting plate geometry is based on wadati benioff zone seismicity and tomography and the 3d thermal structure for both the subducting and overriding plates is constrained by geologic and geophysical observations in models using the composite viscosity a laterally variable mantle viscosity emerges as a consequence of the lateral variations in the mantle flow and strain rate spatially variable mantle velocity magnitudes are predicted with localized fast velocities greater than 80 cm yr close to the slab where the negative buoyancy of the slab drives the flow the same models produce surface plate motions of less than 10 cm yr comparable to observed plate motions these results show a power law rheology i e one that includes the effects of the dislocation creep deformation mechanism can explain both observations of seismic anisotropy and decoupling of mantle flow from surface motion

the moho in extensional tectonic settings insights from thermo mechanical models the lithospheric memory is key for the interplay of lithospheric stresses and rheological structure of the extending lithosphere and for its later tectonic reactivation other important factors are the temporal and spatial migration of extension and the interplay of rifting and surface processes the mode of extension and the duration of the rifting phase required to lead to continental break up are to a large extent controlled by the interaction of the extending plate with slab dynamics the finite strength of the lithosphere has an important effect on the formation of extensional basins this applies both to the geometry of the basin shape as well as to the record of vertical motions during and after rifting we demonstrate a strong connection between the bulk rheological properties of europe s lithosphere and the evolution of some of europe s main rifts and back arc systems the thermo mechanical structure of the lithosphere has a major impact on continental break up and associated basin migration processes with direct relationships between rift duration and extension velocities thermal evolution and the role of mantle plumes compressional reactivation has important consequences for post rift inversion borderland uplift and denudation as illustrated by poly phase deformation of extensional back arc basins in the black sea and the pannonian basin region

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

phreatomagmatic eruptions on the ontong java plateau chemical and isotopic relationship to ontong java plateau basalts the compositions of glass clasts in volcaniclastic rocks recovered from drilling at site 1184 on the eastern salient of the ontong java plateau ojp are investigated using microbeam analytical methods for major minor and trace elements these data are com pared with whole rock elemental and isotopic data for bulk tuff samples and with data from basalts on the high plateau of the ojp three subunits of hole 1184a contain blocky glass clasts thought to represent the juvenile magmatic component of the phreatomagmatic eruptions that generated the volcaniclastic rocks the glass clasts have unaltered centres and are all basaltic low k tholeiites with flat chondrite normalized rare earth element ree patterns their elemental compositions are very similar to the kwaimbaita type and kroenke type basalts sampled on the high plateau each subunit has a distinct glass com position and there is no intermixing of glass compositions between subunits indicating that each subunit is the result of one eruptive phase and that the volcaniclastic sequence has not experienced reworking the relative heterogeneity preserved at site 1184 contrasts with the uniformity of compositions recovered from individual sites on the high plateau and suggests that the eastern salient of the ojp had a different type of magma plumbing system our data support the hypothesis that the voluminous subaerially erupted volcani clastic rocks at site 1184 belong to the same magmatic event as the construction of the main ontong java plateau thus the ojp would have been responsible for volatile fluxes into the atmosphere in addition to chemical fluxes into the oceans and these factors may have influenced the contemporaneous oceanic anoxic event

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

a review of shear and tensile strengths of the malan loess in china abstract loess deposits cover about 640 000 km 2 of china equivalent to about 6 7 of the total area of the country the loess plateau alone covers approximately 317 000 km 2 making it the largest bulk accumulation of loess on the earth land instability hazards in the loess plateau are frequent and varied among which the cracking sliding beng hua in chinese failure is a typical mode cracking sliding failures commonly occur in the malan loess at volumes of the order of 100 m 3 and cause more than 100 deaths per year as the tension in the upper part of the failure plane and shearing sliding along the lower part mobilize cracking sliding failures this failure mode demands an investigation into the tensile and shear strengths this paper presents a review of these properties based on a large collection of literature both tensile and shear strengths of the malan loess decrease with water content and increase with dry density resulting in close multi regression correlations large differences between undisturbed and disturbed tensile strengths of the loess indicate the significance of its structure shear strength of the loess is direction dependent indicating strong anisotropy of the loess structure test method sample size and loading rate all influence the measured tensile strength

influence of physical properties on deformation characteristics of collapsible soils abstract the paper introduces structural pressure values as separation points between elastic and plastic states of any soil including collapsible soils subjected to loading collapsibility of soils is identified as a non elastic deformation the collapse of soils starts when the applied stress exceeds soil structural pressure values initial collapse pressures according to some official documents which depend on change of some soil physical property ies new analytical expressions describing dependence of mechanical characteristics of collapsible soils on soil void ratios and moisture contents are proposed the obtained equations were verified by published test results the algebraic structure of previously obtained empirical expressions between mechanical characteristics of collapsible soils and their physical properties is similar to the structure of theoretically derived equations

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

early tertiary magmatism in ne greenland abstract between 73 and 74 n in ne greenland a lower series of quartz tholeiite lavas is overlain by an upper series which varies from quartz tholeiitic to basanitic the lower series basalts are attributed to large melt fractions produced east of the present outcrop where lithospheric thinning permitted the iceland plume to ascend to shallow levels relocation of eruption sites up to 150 km further west preceeded upper series activity in this small degree melts generated principally from garnet lherzolite facies sources were followed by products of more extensive shallower melting which gave rise successively to the upper series lavas a post upper series basaltic dyke swarm and its associated caldera complex palaeomagnetic observations and 40 ar 39 ar dating suggest that the lower series and part of the upper series erupted in c24r at 58 57 ma rejuvenescent activity at c 32 ma marking the termination of volcanism may relate to decompressive mantle melting associated with late regional uplift

avalanches at the core mantle boundary 1 the partial collapse of topographic structure at the core mantle boundary cmb in avalanches slumps or turbidity flows would cause sudden temperature changes in both the upper core and the lower mantle although such collapses are hypothetical it is interesting to investigate the potential consequences downwelling from such events could disrupt core convection cells and trigger geomagnetic excursions and reversals buoyant sediment from the freezing of the inner core is hypothesized to rebuild the avalanched structures large avalanches could trigger mantle plumes oblique extraterrestrial impacts impart high shear to the cmb and can trigger one or more simultaneous avalanches yielding observed coincidences between craters tektite fields and reversals a triggered avalanche can explain the coincidence between the formation of the largest known volcanic province the ontong java plateau the start of the 35 myr cretaceous geomagnetic quiet period and reported coincidences between large flood basalts and extinctions

crustal thickness anomalies in the north atlantic ocean basin from gravity analysis gravity derived crustal thickness models were calculated for the north atlantic ocean between 76 n and the chain fracture zone and calibrated using seismically determined crustal thickness about 7 of the ocean crust is 7 km thick and is interpreted to have been affected by excess magmatism thin crust probably reflects reduced melt production from relatively cold or refractory mantle at scales of up to hundreds of kilometers along the spreading axis by far the most prominent thick crust anomaly is associated with iceland and adjacent areas which accounts for 57 of total crustal volume in excess of 7 km much smaller anomalies include the azores 8 cape verde islands 6 canary islands 5 madeira 4 and new england great meteor seamount chain 2 all of which appear to be associated with hot spots hot spot related crustal thickening is largely intermittent suggesting that melt production is episodic on time scales of tens of millions of years thickened crust shows both symmetrical and asymmetrical patterns about the mid atlantic ridge mar axis reflecting whether melt anomalies were or were not centered on the mar axis respectively thickened crust at the bermuda and cape verde rises appears to have been formed by isolated melt anomalies over periods of only 20 25 myr crustal thickness anomalies on the african plate generally are larger than those on the north american plate this most likely results from slower absolute plate speed of the african plate over relatively fixed hot spots

intense local toroidal motion generated by variable viscosity compressible convection in 3 d spherical shell strong dynamical effects arising from lateral viscosity variations are found near the heads of fast upwelling plumes approaching the top boundary for unsteady 3 d compressible convection in a spherical geometry at an average rayleigh number of o 106 the temperatures at the center of fast upwelling plume are very hot due to the positive feedback interaction between viscous dissipation and temperature dependence of the viscosity although the laterally averaged ratio of toroidal to poloidal energy varies only around 10 the local toroidal velocities can reach about 40 of the poloidal velocities the strongest toroidal motions always take place right above or close to the fast upwelling plume heads and behave as vortical flows with time this finding may explain the origins of microplate rotation in regions with hotspot activity e g easter island which shows no direct relation to the transform faulting system near the mid ocean ridge

evolution of starting mantle plumes a comparison between numerical and laboratory models abstract laboratory experiments on the initiation and development of mantle plumes suggest the formation of a bulbous plume head that grows in time by entrainment of surrounding material followed by a thin feeder conduit this type of plume is generally not seen in numerical models of mantle convection to better determine the possible differences between numerical and laboratory experiments we have numerically reproduced a published laboratory experiment on plume initiation theoretical predictions for the growth of the plume head have been verified and the experiments allowed for a new determination of a similarity constant models with a constant viscosity fluid show much less entrainment simulations using a plume rising from a thermal boundary layer with an olivine type temperature dependence show that most of the source material is contained in the plume head and that the conduit is largely composed of entrained material simulations with a more realistic non newtonian rheology show that plumes can rise much faster through the mantle which will lead to higher temperatures of the plume head at the earth s surface compared to what experiments with newtonian rheology suggest

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

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

diapiric ascent of magmas through power law crust and mantle there has never been a convincing explanation of the way in which diapirs of molten granite can effectively rise through mantle and crust we argue here that this is mainly because the country rocks have previously been assumed to be newtonian and we show that granitoid diapirs rising through thermally graded power law crust may indeed rise to shallow crustal levels while still molten the ascent velocity of diapirs is calculated through an equation with the form of the hadamard rybczynski equation for the rise of spheres through newtonian ambient fluids this well known equation is corrected by factors dependent on the power law exponent n of the ambient fluid and the viscosity contrast between the drop and the ambient fluid these correction factors were derived from results reported in the fluid mechanical and chemical engineering literature for the ascent of newtonian drops through power law fluids the equation allows calculation of the ascent rates of diapirs by direct application of rheological parameters of rocks the velocity equation is numerically integrated for the ascent of diapirs through a lithosphere in which the temperature increases with depth the depth of solidification of the diapir is systematically studied as a function of the geothermal gradient buoyancy of the body solidus temperature of the magma and rheological parameters of the wall rock the results show that when the wall rock behaves as a power law fluid the diapir s ascent rate increases without a similar increase in the rate of heat loss in this way diapirs rising at 10 to 102 m yr can ascend into the middle or upper crust before solidification strain rate softening rather than thermal softening is the mechanism that allows diapirism to occur at such rates the thermal energy of the diapir is used to soften the country rock only at late stages of ascent the transport of magmas through the lower crust and mantle as diapirs is shown to be as effective as magmatic ascent through fractures

collapsibility and seismic settlement of loess loess collapsibility and seismic settlement have proven to be costly geotechnical hazards in china since collapsibility assessment and mitigation are now conventional practices in chinese design codes some previous efforts have been made to relate collapsibility and seismic settlement in order to simplify assessment of the latter however these works have neglected mechanical difference between the two phenomena in this study the similarity and difference between collapsibility and seismic settlement of loess are analyzed which shows that both are based on the metastable microstructure of loess but their triggering mechanisms are different collapsibility is primarily driven by suction loss whereas seismic settlement is caused by shear failure of the loess matrix during dynamic loading though both of them result in collapse of trellis structure pores consequently the lack of a common mechanism demonstrates there is no tenable physical basis to relate collapsibility and seismic settlement

experimental verification of the fracture density and shear wave splitting relationship using synthetic silica cemented sandstones with a controlled fracture geometry we present laboratory ultrasonic measurements of shear wave splitting from two synthetic silica cemented sandstones the manufacturing process which enabled silica cementation of quartz sand grains was found to produce realistic sandstones of average porosity 29 7 0 5 and average permeability 29 4 11 3 md one sample was made with a regular distribution of aligned penny shaped voids to simulate meso scale fractures in reservoir rocks while the other was left blank ultrasonic shear waves were measured with a propagation direction of 90 to the coincident bedding plane and fracture normal in the water saturated blank sample shear wave splitting the percentage velocity difference between the fast and slow shear waves of 0 5 was measured due to the bedding planes or layering introduced during sample preparation in the fractured sample shear wave splitting corrected for layering anisotropy of 2 72 0 58 for water 2 80 0 58 for air and 3 21 0 58 for glycerin saturation at a net pressure of 40 mpa was measured analysis of x ray ct scan images was used to determine a fracture density of 0 0298 0 077 in the fractured sample this supports theoretical predictions that shear wave splitting sws can be used as a good estimate for fracture density in porous rocks i e sws 100 f where f is fracture density regardless of pore fluid type for wave propagation at 90 to the fracture normal

precise 40ar 39ar age for the initiation of palaeogene volcanism in the inner hebrides and its regional significance sanidine crystals from two tuff layers at the base of the palaeocene eigg lava formation on the isle of muck inner hebrides yield step wise ar release plateau ages of 62 8 0 6 2 and 62 4 0 6 ma using the laser 40 ar 39 ar dating technique these determinations which are in accord with but significantly more precise than earlier results provide a definitive age for the inception of igneous activity in the small isles from stratigraphical inferences the muck ages provide precise maximum constraints on the possible age of skye magmatism and possibly most of the british tertiary volcanic province the new ages significantly predate the oldest ages determined so far for tertiary magmatism in ne greenland but are similar to ages obtained from w greenland and offshore se greenland and support the concept that the initiation of regional volcanism was primarily controlled by lithospheric thin spots the muck sanidine age appears to precisely date the first manifestation of the impingement of the iceland mantle plume on the lithosphere beneath nw scotland

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

two lips and two earth system crises the impact of the north atlantic igneous province and the siberian traps on the earth surface carbon cycle the links between the siberian traps and the end permian mass extinction and between the north atlantic igneous province naip and the paleocene eocene thermal maximum petm demonstrate a critical role for large igneous provinces lips in the disruption of the earth surface carbon cycle escc high precision ages for both volcanic provinces and the associated environmental crises show that in both cases the crisis was contemporaneous with the volcanism the naip comprises two phases the earlier phase 1 c 61 ma and the much more voluminous phase 2 c 56 ma linked to the opening of the ne atlantic the latter triggered the petm the largest cenozoic hyperthermal the siberian traps are significantly more voluminous than the naip and triggered the end permian mass extinction the masses of volcanic co 2 emitted from these provinces may have been much greater than previously suggested as substantial gas may come from intrusive bodies deep within the crust precursory warming due to the accumulation of volcanic co 2 in the atmosphere likely triggered the release of low 13 c methane hydrate although the masses of methane hydrate alone may have been insufficient to account for the observed temperature rises the organic c was likely strongly supplemented by magmatically derived carbon and thermogenic carbon released during emplacement of sills and dykes into c rich sedimentary units more data are required on the volcanic flux rates in order to refine the cause effect relationships between lips and the escc

a first detailed look at the greenland lithosphere and upper mantle using rayleigh wave tomography we present the results of a surface wave study carried out across greenland as part of the glatis greenland lithosphere analysed teleseismically on the ice sheet project rayleigh wave phase velocity dispersion curves were estimated for 45 two station paths across greenland using data from large teleseismic earthquakes the individual dispersion curves show characteristics broadly consistent with those of continental shields worldwide but with significant differences across the greenland landmass reliable phase velocity measurements were made over a period range of 25 160 s providing constraint on mantle structure to a depth of 300 km an isotropic tomographic inversion was used to combine the phase velocity information from the dispersion curves in order to calculate phase velocity maps for greenland at several different periods the greatest lateral variation in phase velocity is observed at intermediate periods 50 80 s where a high velocity anomaly is resolved beneath central southwestern greenland and a low velocity anomaly is resolved beneath southeastern greenland the results of the phase velocity inversion were used to construct localized dispersion curves for node points along two parallel north south profiles in southern greenland these curves were inverted to obtain models of shear wave velocity structure as a function of depth again with the assumption of isotropic structure a similar inversion was carried out for two two station dispersion curves in northern greenland where the resolution of the phase velocity maps is relatively low the models show a high velocity lid structure overlying a zone of lower velocity beneath which the velocity gradually increases with depth the lid structure is interpreted as the continental lithosphere within the lithosphere the shear wave velocity is 4 12 per cent above global reference models with the highest velocities beneath central southwestern greenland however the assumption of isotropic structure means that the maximum velocity perturbation may be overestimated by a few per cent the lithospheric thickness varies from 100 km close to the southeast coast of greenland to 180 km beneath central southern greenland

magma ascent by porous flow porous flow of buoyant liquid through partially molten rock is regarded as the initial transport process leading to magma segregation in the mantle recent work has identified the importance of matrix deformation and compaction in this process we present finite difference calculations based on a generalized form of darcy s law that includes matrix deformation in two dimensional cartesian and cylindrical coordinates we emphasize the existence of solitary wave solutions called magmons these waves are regions of locally high porosity that ascend through regions of low uniform porosity they differ from diapirs where the liquid and matrix ascend together the one dimensional waves that were previously reported are found to be unstable in two dimensions breaking down to form two dimensional waves of circular cross section both the development of the instability and the form of the two dimensional waves are supported by theoretical analysis numerical experiments in which two dimensional waves collide show that the extent to which they are conserved after collision depends on the lateral offset of the waves the conservation can be very good in other cases larger waves appear to grow at the expense of smaller waves although the quantification of the relevant permeability and rheological parameters remains uncertain geophysically plausible estimates suggest wavelengths of kilometers and velocities of centimeters per year in our preliminary assessment of the relevance of these results to igneous processes we find that magmons are unlikely to be important in regions of broad upwelling such as beneath spreading centers here the transport of liquid adjusts to match the supply by melting and compaction processes are not important if liquid is supplied from below to a stable partially molten region of the asthenosphere it is expected to ascend in magmons this may occur beneath oceanic volcanic centers or in subcontinental mantle it is possible that magma ascent in magmons leads to spatial and temporal episodicity of volcanic activity if higher level processes do not obscure this influence the waveform of a magmon ascends faster than the liquid within the magmon so new liquid is taken in from above while original liquid is lost from below consequently magmons can mobilize small degrees of partial melt and deliver it rapidly to the surface

inferences of viscosity from the oceanic geoid indication of a low viscosity zone below the 660 km discontinuity abstract we have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the geoid and tomographic structure beneath the oceans in order to eliminate the gravity signal from problematic masses located below the subduction zones and the continents we have considered only the intermediate degrees of the oceanic geoid l 12 25 a genetic algorithm has been used to determine the family of viscosity models which give the best correlation with the observed geoid our inversion clearly identifies the asthenosphere just below the lithosphere and also confirms the viscosity increase in the lower mantle predicted by previous inferences but suggests that the main viscosity jump occurs at a depth of about 1000 km and not at the usually stated 660 km boundary somewhere in the depth range of 400 1000 km a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere existence of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern with a strong horizontal component in the top part of the lower mantle unfortunately the resolution of the inversion as well as the quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accuracy for the viscosity of this low viscosity channel

a plume head melting under a rifting margin abstract a large igneous province lip in the form of a long narrow band of thickened oceanic crust runs along the atlantic margin of north america abutting the rifted continental shelf we propose that this like many other lips has a mantle plume origin there is evidence that when the central atlantic ocean opened the rift was underlain by the flattened head of a mantle plume and that the rift site had drifted away from the plume tail by the time of the rifting so that the tail took little part in the formation of the lip we carried out numerical simulations in which we rifted the lithosphere over various model plume heads and calculated the volumes of melt produced we found that the thickness and width of the resulting thickened oceanic crust is very sensitive to the thermal structure directly under the rift and the structure of the lithosphere to fit observations of the lip a thin flat plume head is required such a plume head results when a mantle plume with temperature dependent viscosity passes through a significant step reduction in the background mantle viscosity at 660 km depth however an extensive layer of low viscosity under the rift results in a region of thickened crust much wider than the layer is deep by decoupling the mantle flow from the lithosphere to avoid decoupling we propose that there must be significant topography on the lithosphere and the rift site is a region of thinned lithosphere very thick crust next to the margin can be explained by lithospheric necking and the resulting fast initial upflow under the rift

the mechanics of a saturated silty loess and implications for landslides the results from an intensive experimental investigation on a loess that was retrieved from a typical silty loess zone in the north western chinese loess plateau are presented and interpreted triaxial and oedometer tests were performed on intact and reconstituted samples in a saturated condition the soil behavior was found to be strongly affected by structure the compression paths of the intact samples crossed the intrinsic compression line of the reconstituted soil and reached well defined gross yield points after which the compression paths converged towards the intrinsic compression lines two critical state lines were defined for the intact and reconstituted soils in the volumetric plane as a result of a robust element of natural structure comparisons were made with a structured clayey loess retrieved from the south eastern loess plateau it was found that the effects of structure on the behavior of the two loess soils are similar though they are very different in natural properties this indicates that their natural structures might have experienced similar forming processes perhaps related to their common origin catastrophic flowslides commonly occur in the study area and the patterns of behavior observed in the laboratory tests may create some new insights into mechanisms of landslide initiation and subsequent movement as discussed in the study

various influences on plumes and dynamics in time dependent compressible mantle convection in 3 d spherical shell the effects on the dynamics of compressible mantle convection from increasing rayleigh number viscosity stratification and internal heating are investigated in a time dependent 3 d spherical shell model resolution of up to spherical harmonics of degree 192 and 150 higher order finite difference grid points unevenly distributed in the radial direction have been employed surface rayleigh number varies from 5 104 to o 107 the viscosity profiles used range from constant to a depth dependent viscosity profile the adams williamson adiabatic assumption is adopted for describing the equation of state both depth dependences of thermal expansivity and thermal conductivity are included we have focused on the viscosity stratification in the lower mantle the mean temperature profiles are very sensitive to the input parameters such as the amount of internal heating and the dissipation number in the isoviscous model the number of hot plumes decreases for low rayleigh number 5 104 and increases dramatically with higher rayleigh number 106 in the model with a high viscosity zone in the lower mantle the number of plumes is small and does not change too much when the rayleigh number varies from 3 106 to 8 106 in the purely base heated viscously stratified models well organized triple junctions of the downwelling sheets are developed near the surface and interconnected cold walls of downwellings are produced around upwelling plumes the presence of internal heating increases the number of upwelling plumes and breaks up the interconnection of downwelling sheets strong viscous heating in the compressible solutions is found near the top and bottom at the higher values of rayleigh number the laterally averaged value of viscous dissipation is comparable with the chondritic strength of internal heating whether the maximum viscous heating is found in upwelling plumes or downwelling flows depends on the presence of internal heating and the mechanical boundary conditions the effects of compressibility on local structures and plume morphologies can be significant however these compressible effects are moderate on the global patterns

regional uplift gas hydrate dissociation and the origins of the paleocene eocene thermal maximum much of the north atlantic region experienced hundreds of metres of transient uplift during the latest paleocene regional uplift should cause dissociation of gas hydrates stored in marine sedimentary strata and the subsequent release of carbon to the atmosphere should enhance warming through the greenhouse effect we have developed models of the spatial distribution of uplift to match observational constraints from sedimentary basins fringing the north atlantic in order to provide quantitative estimates of the carbon released upon uplift these models are coupled to a simple parameterisation of carbon storage in sub seafloor gas hydrates results of modelling indicate that large quantities of carbon can be released by uplift with some models generating over 2000 gt of carbon release the possibility that latest paleocene regional uplift occurred over the same time interval as onset of the paleocene eocene thermal maximum petm around 30 kyr cannot be ruled out using current stratigraphical observations or mantle convection models in this case the uplift mechanism alone can account for the magnitude and timing of the carbon isotope excursion associated with the petm alternatively if the duration of latest paleocene uplift was closer to the upper bound of around 1 myr or if other paleocene uplift events were equally significant then gas hydrate dissociation upon uplift could have contributed to longer term changes in ocean temperature and carbon isotope composition through the late paleocene these temperature changes in combination with development of the north atlantic land bridge during the latest paleocene culmination of uplift may be able to force rapid changes in ocean circulation and cause phenomena such as the petm

ridge plume interaction in the north atlantic and its influence on continental breakup and seafloor spreading development of the rifted continental margins and subsequent seafloor spreading in the north atlantic was dominated by interaction between the iceland mantle plume and the continental and oceanic rifts there is evidence that at breakup time a thin sheet of particularly hot asthenospheric mantle propagated beneath the lithosphere across a 2500 km diameter region this event caused transient uplift massive volcanism and intrusive magmatism and a rapid transition from continental stretching to seafloor spreading subsequently the initial plume instability developed to an axisymmetric shape with the 100 km diameter central core of the iceland plume generating 30 40 km thick crust along the greenland iceland faeroes ridge the surrounding 2000 km diameter region received the lateral outflow from the plume causing regional elevation and the generation of thicker and shallower than normal oceanic crust we document both long term 10 20 ma and short term 3 5 ma fluctuations in the temperature and or flow rate of the mantle plume by their prominent effects on the oceanic crust formed south of iceland lateral ridge jumps in the locus of rifting are frequent above the regions of hottest asthenospheric mantle occurring in both the early history of seafloor spreading when the mantle was particularly hot and throughout the generation of the greenland iceland faeroes ridge the northern north atlantic is a classic example of continental breakup above a thermal anomaly in the mantle created by a mantle plume the most obvious consequence of this was the generation of huge volumes of basaltic magmas by partial melting of the mantle at the time of continental breakup white mckenzie 1989 most of this magma bled up to the crust where the majority perhaps 60 80 was frozen in the lower crust while the rest was extruded as lava flows and pyroclastic deposits often after undergoing fractionation in crustal magma chambers if the extrusion rates are sufficiently high lava flows may form extensive sub horizontal flood basalts extending across the continental hinterland on rifted continental margins the extrusive lavas are typically expressed as seaward dipping reflector sdr sequences that are readily identifiable on seismic reflection profiles all these features have been found along more than 2 500 km of the rifted continental margins in the northern north atlantic on both the european and the greenland sides furthermore the mantle plume that gave rise to the thermal anomaly in the mantle is still going strong lying beneath iceland at the present day the northern north atlantic is therefore an excellent natural laboratory in which to investigate the interplay between rifting and magmatism because the history of rifting is well recorded from the time of continental breakup in the early tertiary to the seafloor spreading at the present day magnetic anomaly field reversals during the tertiary were frequent and south of iceland there are few large fracture zones in the oceanic crust so the seafloor spreading magnetic anomalies contain a clear record of the rifting history throughout the ocean basin there have also been several well constrained wide angle seismic surveys of the continental margins on both sides of the atlantic e g barton white 1997b smallwood et al 1999 korenaga et al 2000 often with coincident drilling penetrating the volcanic rocks so both the structure of the rifted margins and the timing of the igneous activity are well known in this paper we examine the interaction between the mantle plume and rifting first of the continental lithosphere to form the present continental margins and subsequently of the oceanic lithosphere to form the mid ocean ridge and the land mass of iceland smallwood white rift plume interaction 3

sedimentary response to mantle plumes implications from paleocene onshore successions west and east greenland the uplift history of the upper cretaceous paleocene sedimentary successions underneath the extensive tertiary flood basalts of west and east greenland supports recently described models for the generation of flood basalt provinces in response to rising hot mantle plumes however the sedimentary development indicates that the period of plume related uplift prior to eruption of volcanic rocks was very short 5 m y substantial regional uplift and faulting took place in west and east greenland in the early paleocene to earliest late paleocene resulting in a basinwide unconformity in west greenland the uplift was associated with fluvial peneplanation valley incision and catastrophic deposition in east greenland it was associated with fluvial peneplanation the cumulative uplift in west greenland is probably on the order of several hundred meters whereas in east greenland the observed uplift cannot be quantified in both west and east greenland this phase was shortly followed by rapid major subsidence associated with extensive volcanism it is suggested that this sequence of events reflects the arrival of the north atlantic mantle plume or plumes

pulsing of a focused mantle plume evidence from the distribution of foundation chain hotspot volcanism 1 using the rare case of a hotspot chain crossing a fossil microplate we reveal fundamental mantle plume characteristics by comparing hotspot volcanism in a sequence of contrasting tectonic settings key new 40ar 39ar ages show that the foundation mantle plume pulses hot masses from depth with an apparent periodicity of one myr synchronous magmatism over large distances indicates that masses associated with individual pulses are focused initially into similarly sized zones under the pacific plate since the plume spreading on impact with the lithosphere influences a very wide area apparently unconnected hotspot volcanism can be produced simultaneously across wide swaths often crosscutting seamount chains our model might explain in part much of the midplate volcanism scattered across the pacific seafloor indicating the episodic addition of significantly greater masses of plume material into the upper mantle than suggested by the narrowness of major seamount chains

experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles fractured rocks are known to exhibit seismic anisotropy and shear wave splitting sws sws is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type the presence of partial liquid gas saturation is also known to affect the elastic properties of rocks the combined effect of both fractures and partial liquid gas saturation is still unknown using synthetic silica cemented sandstones with aligned penny shaped voids we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on sws under partial liquid gas saturation conditions the result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model at high to full water saturation values sws decreases as a result of the fluid bulk modulus effect on the quasi shear wave this bulk modulus effect is frequency dependent as a result of wave induced fluid flow mechanisms which would in turn lead to frequency dependent sws this result suggests the possible use of sws for discriminating between full liquid saturation and partial liquid gas saturation

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

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

mantle viscosity derived by genetic algorithm using oceanic geoid and seismic tomography for whole mantle versus blocked flow situations abstract we have applied the genetic algorithm ga technique a nonlinear global optimization method to determine the radial viscosity structure of the mantle from regional geoidal patterns from numerical simulations of 2 d mantle convection we examine the horizontal spectra of the vertical mass flux at 660 km depth and find that for long wavelengths there are minor differences between partially layered convection induced by the phase transitions and mantle convection without any phase transition the differences in the spectra of the vertical mass flux become more prominent at shorter wavelengths this result has led us to study mantle viscosity for the intermediate wavelength geoid from the whole mantle and blocked flow situations in which the appropriate boundary condition is imposed on the radial velocity at 660 km depth in order to confirm the robustness of this study two different density models have been used which were constructed from three tomographic models and appropriate velocity to density scaling relations based on recent results from mineral physics we have analyzed only oceanic geoid spanning between spherical harmonic degree l 12 25 the correlation of the predicted geoid with the observations over the atlantic indian and pacific oceans have been employed as the fitting function in our ga approach which has been modified from the common algorithm in constructing the families of suitable viscosity profiles we have used 100 parents which have been iterated for 100 generations and have been started with 10 different sets of initial parents convergence to acceptable viscosity solutions is obtained for all the three oceans and for both the whole mantle and layered models in some cases multiple viscosity solutions are found acceptable by using the correlation criteria the outstanding feature of these models is the nearly ubiquitous presence of two low viscosity zones one lying under the lithosphere the other right under the bottom of the spinel to perovskite phase change the solutions for the whole mantle model can fit better and are preferred over the solutions with the layered boundary condition which generally result in unrealistic viscosity profiles our results would suggest a more complex mantle viscosity structure which has not been detected previously from geoid signals with longer wavelengths and also reveal the potential difficulties in treating the dynamical boundary condition at the 660 km discontinuity

pulsating diapiric flows consequences of vertical variations in mantle creep laws abstract recent laboratory work has suggested that the rheology of the lower mantle may be newtonian we have studied the time dependent dynamics of plumes interacting with a rheological interface separating an upper non newtonian mantle and a newtonian lower mantle pulsating diapiric structures with fast time scales are promoted by the interaction of the rising plumes with this rheological boundary surface heat flow signals are discernible as pulses which remain relatively stationary they correlate well with the localized upwellings just below the surface the presence of a mobile lithosphere from increasing the non newtonian power law index helps to produce a large scale circulation in the upper mantle which draws large hot patches away from nearby upwellings in our calculations the resultant averaged effective viscosity of the non newtonian upper mantle is about two orders of magnitude lower than that of the newtonian lower mantle a viscously stratified newtonian model produces more incoherent and broad scale diapiric structures and is less efficient for generating sharply varying time dependent thermal signatures

pore fluid viscosity effects on p and s wave anisotropy in synthetic silica cemented sandstone with aligned fractures ultrasonic 500 khz p and s wave velocity and attenuation anisotropy were measured in the laboratory on synthetic octagonal shaped silica cemented sandstone samples with aligned penny shaped voids as a function of pore fluid viscosity one control blank sample was manufactured without fractures another sample with a known fracture density inline image measured from x ray ct images velocity and attenuation were measured in four directions relative to the bedding fabric introduced during packing of successive layers of sand grains during sample construction and the coincident penny shaped voids fractures both samples were measured when saturated with air water viscosity 1 cp and glycerin 100 cp to reveal poro visco elastic effects on velocity and attenuation and their anisotropy the blank sample was used to estimate the background anisotropy of the host rock in the fractured sample the bedding fabric was found to show transverse isotropy with shear wave splitting sws of 1 45 1 18 i e for s wave propagation along the bedding planes in the fractured rock maximum velocity and minimum attenuation of p waves was seen at 90 to the fracture normal after correction for the background anisotropy the fractured sample velocity anisotropy was expressed in terms of thomsen s weak anisotropy parameters a theory of frequency dependent seismic anisotropy in porous fractured media was able to predict the observed effect of viscosity and bulk modulus on and in water and glycerin saturated samples and the higher and values in air saturated samples theoretical predictions of fluid independent are also in agreement with the laboratory observations we also observed the predicted polarisation cross over in shear wave splitting for wave propagation at 45 to the fracture normal as fluid viscosity and bulk modulus increases

quantifying transient mantle convective uplift an example from the faroe shetland basin the icelandic mantle plume generated maximum uplift in early paleogene times the faroe shetland basin which fringes the north atlantic margin of europe was close to the center of early paleogene icelandic plume activity three dimensional seismic reflection data from the faroe shetland basin reveal sedimentary geometries that allow a phase of transient uplift to be accurately reconstructed and quantified close to the paleocene eocene boundary circa 56 ma rapid uplift resulted in fluvial incision into marine sediments this unconformity was buried by nonmarine sediments recording the decay of transient uplift relief on the unconformity was 550 m constraining the minimum amount of surface uplift some 60 m of this uplift can be attributed to the isostatic response to erosional unloading tectonic uplift of over 490 m peaked and decayed within 3 ma rates of water loaded tectonic subsidence following peak uplift are several times greater than maximum expected postrift subsidence rates the amplitude and duration of this transient effect is best explained by a mantle convective phenomenon we suggest that a region of hot plume material flowed laterally beneath the lithosphere producing transient uplift which decayed when plume material was advected farther away our analysis suggests that under certain circumstances stratigraphic records can yield valuable quantitative information about aspects of mantle convective circulation

lithospheric memory state of stress and rheology neotectonic controls on europe s intraplate continental topography abstract to date research on neotectonics and related continental topography development has mostly focused on active plate boundaries characterized by generally high deformation rates the intraplate sedimentary basins and rifts of the northern alpine foreland are associated with a much higher level of neotectonic activity than hitherto assumed seismicity and stress indicator data combined with geodetic and geomorphologic observations demonstrate that europe s intraplate lithosphere is being actively deformed this has major implications for the assessment of its natural hazards and environmental degradation the lithosphere of the northern alpine foreland has undergone a polyphase evolution with an intense interplay between upper mantle thermal perturbations and stress induced intraplate deformation that points to the importance of lithospheric folding of the thermally weakened lithosphere in this paper we address relationships between deeper lithospheric processes neotectonics and surface processes in the northern alpine foreland with special emphasis on tectonically induced topography the objectives are to quantify the effects of ongoing alpine collision and atlantic ridge push on the intraplate deformation in europe and its impact on topography evolution and related natural hazards this paper reviews the four dimensional topographic evolution of the european lithosphere through a multi disciplinary approach linking geology geophysics and geotechnology until now research on neotectonics and related topography development of intraplate regions has received little attention our study examines a number of selected natural laboratories in continental europe from orogen through platform to continental margin these natural laboratories include the carpathians pannonian system the northwest european platform iberia and the atlantic continental margin we focus on lithosphere memory and neotectonics with special attention to the thermo mechanical structure of the lithosphere mechanisms of large scale intraplate deformation late neogene anomalies in subsidence and uplift and links with surface processes and topography evolution

dynamical consequences of depth dependent thermal expansivity and viscosity on mantle circulations and thermal structure the effects of both depth dependent thermal expansivity and depth dependent viscosity on mantle convection have been examined with two dimensional finite element simulations in aspect ratio ten boxes surface rayleigh numbers between 107 and 6 107 have been considered the effects of depth dependent properties acting singly or in concert are to produce large scale circulations with a few major upwellings the interior of the mantle is cooled by the many cold instabilities which are slowed down and eventually swept about by the large scale circulation the interior temperature of the mantle can be influenced by the trade off between depth dependent properties and internal heating for chondritic abundance of internal heating and depth dependent thermal expansivity the viscosity increase across the mantle can be no greater than a factor of around ten in order to keep the lower mantle adiabatic the thermal contrasts between the cold blobs and the surrounding mantle are strongly reduced by depth dependent properties whereas the lateral differences between the hot upwelling and the ambient lower mantle can be significant over several hundred degrees depth dependent properties also encourage the formation of a stronger mean flow in the upper mantle which may be important for promoting long term polar motions

ultrafast upwelling bursting through the upper mantle a high resolution calculation of strongly time dependent thermal convection in the upper mantle with non newtonian temperature dependent rheology shows that for an effective rayleigh number of around 106 extremely fast upwellings at times exceeding 10 m yr can be generated a few hundred kilometers below the lithosphere there is a clear separation of timescales between this fast jet and the more slowly convecting mantle within this fast vertical shear layer is embedded a thermal boundary layer with a width of the order of 50 km the development of the fast non newtonian upwelling is characterized by the growth of the plume head to a large enough size before the plume takes off rapidly at a depth of around 350 km upon impinging the base of the lithosphere this fast plume thins the lithosphere and the flow then becomes a horizontally moving hot sheet extending out for around 1000 km this scenario is found to repeat itself at the same location about 10 myr after the first plume impingement

seismic tomogram of the earth s mantle geodynamic implications recent seismic tomography of the earth s mantle has revealed a large scale pattern of mantle convection comprising upwelling columnar plumes in the pacific and africa and downwelling planar sheets along the circum pacific upwelling and downwelling occur most extensively under the south pacific and west pacific respectively high resolution image of plate subduction has been obtained from the dense seismic networks around japan japanese seismologists are in the best position to resolve the internal structure of downwelling current as an integral part of the whole convection system

contrasting rifted margin styles south of greenland implications for mantle plume dynamics abstract we present new and reprocessed seismic reflection data from the area where the southeast and southwest greenland margins intersected to form a triple junction south of greenland in the early tertiary during breakup at 56 ma thick igneous crust was accreted along the entire 1300 km long southeast greenland margin from the greenland iceland ridge to and possibly 100 km beyond the triple junction into the labrador sea however highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest greenland margin at the same time thus a transition from a volcanic to a non volcanic margin over only 100 200 km is observed magmatism related to the impact of the iceland plume below the north atlantic around 61 ma is known from central west and southeast greenland the new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction the extent of initial plume related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere plume mantle is channelled to great distances provided that significant melting does not take place melting causes cooling and dehydration of the plume mantle the associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift we further suggest that thick archaean lithosphere blocked direct flow of plume material into the magma starved southwest greenland margin while the plume was free to flow into the central west and east greenland margins the model is consistent with a plume layer that is only moderately hotter 100 200 c than ambient mantle temperature and has a thickness comparable to lithospheric thickness variations 50 100 km lithospheric architecture the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism

formation and development of the structure of collapsing loess in china abstract gao g 1988 formation and development of the structure of collapsing loess in china eng geol 25 235 245 it is well known that in china the regional variations in microstructure of loess are in phase with the overall tendency for collapsibility gradually decreasing from the northwest to the southeast these differences are consistent with variations in climatic conditions this indicates that the structure of collapsing loess is closely related to the climatic condition and geological environment based on the microstructure analyses it can be assumed that the formation and development of the structure of collapsing loess in china has undergone five stages the loessization stage the formation stage the development stage the degeneration stage and the clayization stage the five stages of loess regularly exist in different regions of the loess plateau this also shows that under the modern climatic condition the collapsing loess in the northwest of the loess plateau in china is still developing however the feature of collapsing loess is gradually disappearing in the southeast of the loess plateau

ultrafast mantle plumes and implications for flood basalt volcanism in the northern atlantic region abstract recent geochemical and geochronological data on paleocene flood basalts from west greenland se greenland and the british isles show that volcanic activity at these widely separated locations commenced nearly simultaneously around 61 62 ma ago and that the duration of this initial phase of flood basalt magmatism was of the order of a few million years or less a small fast moving upper mantle plume which rapidly spreads out horizontally on encountering the base of the lithosphere appears to be a viable mechanism to explain these observations however in order to reconcile the idea of an ultrafast plume with the observed plate velocities a physical mechanism is needed for inducing a separation of timescales between the plume speed and the surrounding mantle circulation thermal convection with a non newtonian temperature and depth dependent rheology provides such a mechanism wherein extremely fast plumes ascending at velocities between one to tens of meters per year can be produced in an otherwise slowly convecting mantle moving at cm yr this transport mechanism is capable of bringing up very hot matter from the transition zone to the lithosphere the fast upward velocities lead to the production of high viscous heating rates surrounding the plume upon impinging the lithosphere it is thus possible to explain the near simultaneous onset of magmatism in west greenland se greenland and the british isles in the early tertiary by a fast moving mantle plume spreading out horizontally with a velocity of around 0 5 m yr for resolving numerically the thermal mechanical state of these strongly time dependent mantle flows extremely high spatial resolution on the kilometer scale is required finally we suggest the possibility that the source of the fast upper mantle plume under iceland may be rooted in the lower mantle this is consistent with the recent findings by seismic tomography of a deep mantle plume under iceland

fast plumeheads temperature dependent versus non newtonian rheology in a series of two dimensional cartesian simulations with high resolution on the order of 2 km we have compared for the same effective rayleigh number around 106 the dynamics between newtonian and non newtonian rheologies a viscosity contrast due to temperature of 1200 has been employed throughout with surface dissipation numbers varying between 0 05 and 0 2 peak velocities for non newtonian plumes are at least 50 times faster than the peak newtonian velocities and reach magnitudes in excess of m yr upon impinging the surface plume head structures of the non newtonian rheology contain much greater complexity in the thermal mechanical structure than for newtonian plumes

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

observations of fluid dependent shear wave splitting in synthetic porous rocks with aligned penny shaped fractures p and s wave velocity and attenuation coefficients accurate to 0 3 and 0 2 db cm respectively were measured in synthetic porous rocks with aligned penny shaped fractures using the laboratory ultrasonic pulse echo method shear wave splitting was observed by rotating the s wave transducer and noting the maximum and minimum velocities relative to the fracture direction a block of synthetic porous rock of fracture density 0 0201 0 0068 and fracture size 3 6 0 38 mm measured from image analysis of x ray ct scans was sub sampled into three 20 30 mm long 50 mm diameter core plugs oriented at 0 45 and 90 to the fracture normal transversely isotropic symmetry axis full waveform data were collected over the frequency range 500 1000 khz for both water and glycerin saturated cores to observe the effect of pore fluid viscosity at 1 cp and 100 cp respectively the shear wave splitting observed in the 90 core was 2 15 0 02 for water saturated and 2 39 0 02 for glycerin saturated in agreement with the theory that suggests that the percentage splitting should be 100 times the fracture density and independent of the saturating fluid in the 45 core by contrast splitting was 0 00 0 02 for water saturation and 0 77 0 02 for glycerin saturation this dependence on fracture orientation and pore fluid viscosity is consistent with the poro visco elastic theory for aligned meso scale fractures in porous rocks the results suggest the possible use of shear or converted wave data to discriminate between fluids on the basis of viscosity variations

engineering properties and zoning of loess and loess like soils in china the widely distributed loessic soils of china are described concentrating on the collapsible soils of the upper strata of the loessic series a general trend of gradual steady reduction in the tendency to collapse on wetting is found going in the direction from northwest to southeast cases of damage and flooding tests of pits are cited for illustration results of electron microscope studies of structural features of loess specimens from different locations are compared with the engineering behavior especially the susceptibility to self subsidence and degree of collapsibility in view of this trend and the differences between loessic soils of different regions a preliminary zoning of the loessic soils was made resulting in seven zones as shown on a schematic map

dynamical effects of a temperature and pressure dependent lower mantle rheology on the interaction of upwellings with the transition zone we have examined the consequences of a temperature and pressure dependent lower mantle viscosity on the interaction of upwellings with the two major phase transitions the viscosity used is only temperature dependent in the upper mantle but assumes both temperature and pressure dependences in the lower mantle the activation enthalpy increases with depth dependence scaled after experimental melting curves of lower mantle components we have employed a two dimensional cartesian box with an aspect ratio of five and a mantle layer thickness of 2900 km using an extended boussinesq model with a depth dependent thermal expansivity latent heat release and viscous dissipation have been included in the temperature equation we have considered models both without and with a viscosity jump a factor of 8 across the 660 km boundary the range of averaged rayleigh numbers ra considered lies between 2 106 and 107 a major effect of this temperature and pressure dependent rheology is the stabilization of the upwelling plumes and the increase of viscous heating in the bottom portion of these plumes the impingement of these heated plumes at the endothermic phase boundary causes the development of secondary plumes to rise into the upper mantle shear heating together with the latent heat release produces large temperature increases in the plumes passing through the transition zone which may cause melting in the deep upper mantle although the average rayleigh number in the case of temperature and pressure dependent viscosity is around a factor of 5 lower than in a case with purely temperature dependent viscosity the resulting flow is only slightly less layered which is contrary to the earlier finding of christensen and yuen 1985 the tendencies for the development of secondary plumes and high temperatures inside the upwellings are enhanced by the viscosity jump at the 660 km boundary in this situation few nearly steady vigorous upwellings exist in the lower mantle while in the upper mantle vigorous time dependent convection takes place for an averaged rayleigh number of 107 the system becomes layered with many thin plumes appearing in the lower mantle

gravity inferred crustal thickness of greenland abstract greenland s crust contains clues for understanding the dynamics of the earth s second largest ice sheet as well as the iceland hotspot over the past 100 ma however our knowledge of the sub ice crust is very limited due to the paucity of outcrops and seismic observations particularly in central greenland in order to augment these sparse constraints we modeled terrain and airborne free air gravity data which provide estimates for moho and related crustal thickness these estimates were obtained from the inversion of isostatically corrected terrain effects where the computed gravity effects of the terrain s rock water and ice components were subtracted from the terrain correlated free air anomalies the terrain correlated free air components were estimated from the correlation spectrum between the free air anomalies and the terrain s gravity effects which are assumed to reflect isostatic disturbances of the crustal terrain the results indicated an average crustal thickness of about 37 km for greenland with greater values by 5 10 km in the southern greenland archean blocks and caledonian foldbelt and comparably lower values in northern greenland the thinner crustal thickness estimates in the north are strongly correlated with the faster basal melt regions of the ice sheet and glaciers the crustal thickness map provides constraints for an improved interpretation of the iceland hotspot track in northern greenland at about 100 70 ma

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

heavy rainfall triggered loess mudstone landslide and subsequent debris flow in tianshui china abstract continuous and heavy precipitation triggered a large loess landslide at the southern dagou village of mapaoquan town tianshui city gansu province of china on july 21 2013 the landslide debris rapidly turned into a debris flow that was deposited in the ravine mouth with the volume of 1 9 105 m3 detailed field mapping three dimensional laser scanning aerial photograph interpretation and laboratory tests were carried out to study the formation and moving characteristics of the landslide and subsequent debris flow the results showed that 1 the peak flow velocity and peak discharge of the debris flow were estimated to be approximately 7 2 m s and 730 m3 s respectively the velocity had a tendency to first increase and then decrease from the head to entrance in the dagou gully 2 the analysis of rainfall conditions showed that the effective antecedent rainfall within 7 days and hourly rainfall intensity which triggered the slide debris flows was 239 mm and 20 mm h respectively compared with the critical rainfall in this area the effective antecedent rainfall was found to be more significant in triggering this event 3 the mean and effective particle sizes were approximately 0 73 1 3 mm and 0 036 0 087 mm respectively and had the same distribution along the gully as the velocity which confirmed the process of the variation in the flow velocity 4 the scale amplification was very obvious in the formation and moving process of the slide debris flows

global mantle shear velocity model developed using nonlinear asymptotic coupling theory we present a three dimensional shear velocity model of the whole mantle developed using s h waveform data the model is expressed horizontally in terms of spherical harmonics up to degree 12 and vertically in terms of legendre polynomials up to degrees 5 and 7 in the upper and lower mantle respectively what distinguishes this model from other tomographic models published to date is 1 the theoretical normal mode based wave propagation approach where we include across branch mode coupling terms in order to model the body wave sensitivity to structure along the path more accurately 2 the wave packet weighting scheme which allows to balance contributions from high amplitude and low amplitude phases increasing the resolution in some parts of the mantle we also relax the constraints on the moho depth which is allowed to vary in the inversion thus absorbing some uncertainties in crustal structure the resulting model is generally in good agreement with other recent global mantle s velocity models and with some regional models the rms profile with depth has more power than other models in the upper mantle lower mantle transition region and the zone of increased power and low degree structure near the base of the mantle is confined to the last 500 km in depth this model provides a particularly good fit to the non hydrostatic geoid through harmonic degree 12 79 variance reduction as well as good fits to observed splitting functions of s velocity sensitive mantle modes indicating that both large scale and small scale features are really well constrained