crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

development and applications of double difference seismic tomography double difference dd tomography is a generalization of dd location it simultaneously solves for the three dimensional velocity structure and seismic event locations dd tomography uses a combination of absolute and more accurate differential arrival times and hierarchically determines the velocity structure from larger scale to smaller scale this method is able to produce more accurate event locations and velocity structure near the source region than standard tomography which uses only absolute arrival times we conduct a stability and uncertainty analysis of dd tomography based on a synthetic data set currently three versions of the dd tomography algorithms exist tomodd tomofdd and tomoadd tomodd assumes a flat earth model and uses a pseudo bending ray tracing algorithm to find rays between events and stations while tomofdd uses a finite difference travel time algorithm and the curvature of the earth is considered both codes are based on a regularly distributed inversion grid with the former for a local scale and the latter for a regional scale in contrast tomoadd adapts the inversion mesh to match with the data distribution based on tetrahedral and voronoi diagrams we discuss examples of applying dd tomography to characterize fault zone structure image high resolution structure of subduction zones and determine the velocity structure of volcanoes

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

convection experiments and the driving mechanism the problem of mantle convection is discussed in light of laboratory and numerical experiments since wegener s time our knowledge of earth behaviour has increased so that we are in a better position to explain continental drift the explanation of this phenomenon must inevitably include mantle convection the problem has three aspects the physical laws the boundary conditions and the material we find that a likely model is high rayleigh number convection in a medium of temperature pressure and stress dependent rheology in this model we expect a strong and heavy upper thermal boundary layer and a fluid and light lower boundary layer the upper one consists of the lithospheric plates which highly organize the large scale circulation while the lower one at the core mantle boundary becomes unstable in the form of diapirs plumes or blobs in a fashion rather independent of the large scale circulation this model has the potential of reconciling the conflicting views of horizontal and vertical tectonics

dynamical effects on the core mantle boundary from depth dependent thermodynamical properties of the lower mantle a common assumption in modelling dynamical processes in the lower mantle is that both the thermal expansivity and thermal conductivity are reasonably constant recent work from seismic equation of state leads to substantially higher values for the thermal conductivity and much lower thermal expansivity values in the deep mantle the dynamical consequences of incorporating depth dependent thermodynamic properties on the thermal mechanical state of the lower mantle are examined with the spherical shell mean field equations we find that the thermal structure of the seismically resolved anomalous zone at the base of the mantle d is strongly influenced by these variable properties and in particular that the convective distortion of the core mantle boundary cmb is reduced with the decreasing thermal expansivity such a reduction of the dynamically induced topography from pure thermal convection would suggest that some other dynamical mechanism must be operating at the cmb

variability in flow and temperatures within mantle subduction zones 1 a series of laboratory experiments is used to model three dimensional aspects of flow in subduction zones and the consequent temperature variations in the slab and overlying mantle wedge the effects of longitudinal rollback and slab steepening components of motions are considered along with different thicknesses of the over riding lithosphere the results show that the style of plate sinking influences the evolution of subduction zones both in terms of the speed orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab in the simplest case of longitudinal sinking without rollback motion velocities in the mantle wedge are 30 40 of the downdip plate speed return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds and there is no mass flux around the edges of a slab segment of finite width rollback subduction leads to flow both around and beneath the sinking slab with larger velocities in the wedge up to 150 of the slab speed and flow focused toward the center of the plate segment rollback subduction in which either the trench migrates or the plate steepens with time induces shallow and steep wedge return flow trajectories respectively the thermal evolution of the plate is strongly influenced by sinking style and rate highest temperatures are along the edges of the slab for longitudinal sinking but along the centerline of the slab segment for rollback motion slab surface temperatures when scaled to the mantle are higher than those given by previous two dimensional numerical models and are consistent with recent observational and experimental data on melt compositions

role of the lithosphere in mantle convection plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath and plate formation and subduction account for most of the heat transport from the earth s interior correspondingly mantle convection models must incorporate a stiff but mobile boundary layer like the lithosphere before they can reproduce the main features of mantle convection a relatively easy way to accomplish this in numerical models is to combine a temperature dependent viscosity with an imposed piecewise constant surface velocity boundary condition it is shown how surface heat flux topography gravity and geoid but not plate velocities or stresses can then be derived numerical models confirm that a lithosphere has a first order effect on the underlying flow structure for internally heated models approx imating the case of whole mantle convection the calculated surface signatures are in first order agreement with observations a level of empirical success which hitherto has not been approached by models of mantle convection companion papers exploit the observations more fully to constrain the main features of mantle convection

geoid and topography of earth like planets a comparison between compressible and incompressible models for different rheologies abstract a systematic study of 2d axisymmetric spherical shell models of compressible and incompressible mantle convection with constant and variable viscosity and constant and depth dependent thermodynamic properties is presented to account for compressibility effects we employ the anelastic liquid approximation in the case of variable viscosity an arrhenius law with strongly temperature and pressure dependent viscosity is considered we show that assuming compressible convection with depth dependent thermodynamic properties strongly influence the geoid undulations using compressible convection with constant thermodynamic properties is physically inconsistent and may lead to spurious results for the geoid and convection pattern in addition we examine the impact of compressibility as well as different rheologies on the power law relation that connects the nusselt number to the rayleigh number we discover that the power law index of the nu ra relationship is controlled by the rheology independent of which approximation is used instead the bound of this relation is controlled by a combination of different approximation and rheology

two crustal low velocity channels beneath se tibet revealed by joint inversion of rayleigh wave dispersion and receiver functions abstract competing geodynamic models such as rigid block extrusion continuous deformation and the mid lower crustal flow have been proposed to describe the growth and expansion of eastern tibet however the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure on the basis of joint inversion of rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array we have obtained a high resolution 3d image that reveals the distribution of low velocity zones lvzs with unprecedented clarity the prominent feature of our model is two low velocity channels that bound major strike slip faults in se tibet and wrap around the eastern himalaya syntaxis consistent with the clockwise movement of crustal material in this region most large earthquakes in this region occurred in the boundaries of the lvzs we propose that ductile flow within these channels in addition to shear motion along strike slip faults played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of tibet in the cenozoic

contemporary crustal deformation around the southeast borderland of the tibetan plateau 1 we derive a detailed horizontal velocity field for the southeast borderland of the tibetan plateau using gps data collected from the crustal motion observation network of china between 1998 and 2004 our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes separated by strike slip and transtensional faults most notably the regional deformation includes 10 11 mm yr left slip across the xianshuihe fault 7 mm yr left slip across the anninghe zemuhe xiaojiang fault zone 2 mm yr right slip across a shear zone trending northwest near the southern segment of the lancang river fault and 3 mm yr left slip across the lijiang fault deformation along the southern segment of the red river fault appears not significant at present time the region south and west of the xianshuihexiaojiang fault system whose eastward motion is resisted by the stable south china block to the east turns from eastward to southward motion with respect to south china resulting in clockwise rotation of its internal subblocks active deformation is detected across two previously unknown deformation zones one is located 150 km northwest of and in parallel with the longmenshan fault with 4 6 mm yr right slip and another is continued south southwestward from the xiaojiang fault abutting the red river fault with 7 mm yr left slip while both of these zones are seismically active the exact locations of faults responsible for such deformation are yet to be mapped by field geology comparing our gps results with predictions of various models proposed for tibetan plateau deformation we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger highly fragmented upper crust

joint inversion of body wave arrival times and surface wave dispersion for three dimensional seismic structure around safod we incorporate body wave arrival time and surface wave dispersion data into a joint inversion for three dimensional p wave and s wave velocity structure of the crust surrounding the site of the san andreas fault observatory at depth the contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations we find that the trade off between fitting the two data types controlled by the weighting defines a clear optimal solution varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type all the acceptable solutions yield structures with similar primary features but the smaller scale features change substantially when there is a lower relative weight on the surface wave data it appears that the solution over fits the body wave data leading to a relatively rough vs model whereas for the optimal weighting we obtain a relatively smooth model that is able to fit both the body wave and surface wave observations adequately

joint inversion for three dimensional s velocity mantle structure along the tethyan margin abstract for purposes of studying the lateral heterogeneity as well as for ultimately predicting seismograms for this region we construct a new 3 d s velocity model by jointly inverting a variety of different seismic data we jointly invert regional waveforms surface wave group velocity measurements teleseismic s arrival times and crustal thickness estimates from receiver functions refraction lines and gravity surveys these data types have complementary resolving power for crust and mantle structures vertical and lateral variations shallow and deep mantle features local and global structure therefore a joint inversion of these data sets might help unravel the complexity of this tectonically diverse area these measurements are made from a combination of mantle investigation of the deep suture between europe and africa midsea program for array seismic studies of the continental lithosphere passcal geoscope geofon global seismographic network gsn international deployment of accelerometeres ida mednet national networks and local deployments throughout the study region which extends from the western mediterranean region to the hindu kush and encompasses northeastern africa the arabian peninsula the middle east and part of the atlantic ocean for reference we have fitted the waveforms of regional s and rayleigh waves from over 3800 seismograms using partitioned waveform inversion we include over 3000 crustal thickness estimates from receiver functions gravity measurements and refraction profiles we include rayleigh wave group velocities for hundred thousands of paths transecting the region we have over 3000 teleseismic s arrival times measured through cross correlation and over 170000 from picks originally reported to the international seismological centre isc

convection in a variable viscosity fluid newtonian versus power law rheology abstract a number of finite element calculations of convection in a variable viscosity fluid have been carried out to investigate the effects of non newtonian flow when rheology is also subject to a strong temperature and pressure influence a variety of cases has been studied in the range of effective rayleigh numbers between 104 and 106 including different modes of heating and a range of values for activation energy and activation volume power law creep with a stress exponent of 3 turns out to lead to considerably different flow pattern and heat transfer properties than newtonian rheology in general the effect is to reduce viscosity contrasts imposed by p t dependence which can lead in some circumstances to the mobilisation of otherwise stagnant regions within the cell the properties of non newtonian flow can be closely imitated by a newtonian fluid with a reduced value of the activation enthalpy bh with b 0 3 0 5 it appears possible that non newtonian rheology plays a key role in determining the convective style in a planetary mantle

late quaternary activity of the zemuhe and xiaojiang faults in southwest china from geomorphological mapping abstract there has been limited detailed geomorphological research on the kangding fault system located along the eastern margin of the tibetan plateau despite the system containing active faults with high slip rates fault related landforms such as offset streams were mapped at selected sites along the zemuhe and xiaojiang faults in the central and southern sections of the kangding fault system the results permitted assessment of the mode and rate of late quaternary fault movement based on more comprehensive information than used in previous studies the left lateral strike slip of these faults reflects the large scale clockwise rotation of the southern tibetan plateau the estimated strike slip rate of the zemuhe fault is ca 5 9 mm yr 1 whereas that of the xiaojiang fault is ca 14 22 mm yr 1 these rates and the known slip rates of the adjacent faults indicate that in the northern and southern sections of the kangding fault system the regional tectonic stress from the clockwise rotation has been released mainly along the major faults whereas the stress in the central section has been released by both fault movement and deformation in surrounding areas the latter mechanism reflects a more complex geological structure in the central system of distinct thrusts and folds late quaternary vertical deformation along the zemuhe and xiaojiang faults has mostly resulted from local stresses associated with strike slip movement although the zemuhe fault has also undergone slow but consistent normal faulting since the early quaternary due to its orientation oblique to the direction of the clockwise rotation

geoid and topography associated with sublithospheric convection negligible contribution from deep currents abstract the sources of geoid and topography anomalies in a convecting upper mantle with realistic temperature and pressure dependent viscosity are studied the results differ from those obtained in previous studies with layered depth dependent viscosity for a newtonian viscosity the topography and the geoid anomalies coming from below the low viscosity zone i e deeper than 250 km amount to 30 m and 10 cm respectively while they are reduced to 10 m and 10 cm for a non newtonian viscosity it represents at most 6 of the complete signal and affects very little the geoid over topography ratio the weakness of the contribution from the lower part of the cell is due to both small kernels and small lateral temperature variations at depth yet a linear regression of the complete geoid signal versus topography yields an apparent compensation depth shallower than the base of the lithosphere the damping of the vertical stress through a lithosphere with a depth dependent viscosity implies a departure from isostasy and is the cause of the shallow apparent compensation depths found in our model

mantle convection with spherical effects results of a similarity theory for spherical mantle convection are presented the single mode mean field equations are analyzed for convection which is so vigorous that temperature disturbances become localized in thin thermal boundary layers our purpose is to study effects of spherical geometry density interfaces heat source distribution and cell size steady state solutions are found for isoviscous spherical shells in which the field of motion is spatially periodic in a single spherical harmonic degree calculations are carried out over the range 2 l 40 and for various fractions of internal versus base heating three configurations are examined 1 convection in a single layer of cells extending through the whole mantle 2 convection in two layers separated by a density interface at 670 km depth and 3 convection in a single layer terminating at 670 km results of these calculations are used to give estimates of surface horizontal velocities in terms of the heat loss viscosity stratification amount of internal heating and depth of circulation the surface velocity is most strongly affected by the thickness of the convecting shell deep mantle convection can achieve surface velocities which agree with observed plate speeds while convection restricted to the upper mantle does not at least on the scale of the major plates the temperature distribution is strongly affected by the spherical geometry and by the presence of density interfaces the principal difference between convection in one and two layers is that the latter produces a hot lower mantle while the former produces a warm one

compressible convection with constant and variable viscosity the effect on slab formation geoid and topography two dimensional cartesian finite difference models of compressible convection with constant and variable viscosity and fixed bottom temperature are presented density variations according to the adams williamson equation of state are included in the case of constant viscosity convection viscous and adiabatic heating damp the flow compressible density variations hardly influence geoid undulations however the lower thermal boundary layer becomes thinner and the mean cell temperature increases in the case of variable viscosity the nonlinear coupling between compression adiabatic and viscous heating and a temperature pressure and stress dependent rheology leads to important consequences the upwelling flow broadens and plumes are retarded the flow strongly concentrates toward the downwelling region and becomes mechanically decoupled from the interior of the cell by a low viscosity region this mechanism seems to be important for the formation of subducting slabs extrapolated to mantle conditions two low viscosity regions are predicted flanking the slab on either side and inhibiting an early dispersal and mixing of slab material into the mantle this process might be aided by an increase of negative buoyancy forces with depth as observed in the models further results are as follows increasing the dissipation number in variable viscosity convection may either damp or speed up convection depending on the rheology models with internal heating and a fixed bottom temperature show that the threshold to time dependent variable viscosity convection is drastically reduced if the anelastic liquid approximation is applied instead of the extended boussinesq approximation

the influence of second scale convection on the thickness of continental lithosphere and crust abstract the effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies newtonian non newtonian and temperature and pressure dependent heat fluxes and heating modes bottom versus internal heating the stagnant regions near the top of the models are identified with the lithosphere we distinguish a seismic and thermal lithosphere controlled by temperature an elastic lithosphere controlled by viscosity and a mechanical lithosphere controlled by strain rate strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop the thickness of the elastic lithosphere is about two thirds of the mechanical lithosphere while the ratio of the elastic to the thermal lithosphere varies between 0 4 and 0 8 the time dependence of some models with internal heating and a surface heat flow of 20 mw m 2 is characterized by long periods of the order of 1 ga close to steady state and short periods 100 ma of changing cell patterns and lithospheric thicknesses models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold slowly downwelling convective flow rather than being firmly attached to the lithosphere the missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the moho observed lithospheric thickness variations in europe and their minimum to maximum spacing agree well with the models suggesting sublithospheric convection to be the cause compared to other observed features thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection relatively low viscosities in the lower crust may lead to a long term decoupling of surface topography from moho topography and mantle dynamics the moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 ma while surface topography may flatten out such a mechanism may be important in regions with a deep moho such as fennoscandia or with a shallow moho such as the pannonian basin neither region showing a pronounced surface topography

radial anisotropy in the crust of se tibet and sw china from ambient noise interferometry 1 we use rayleigh and love wave green s functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of the sichuan basin and around the eastern himalaya syntaxis in agreement with previous studies low velocity zones are ubiquitous in the mid lower crust with substantial variations both laterally and vertically discrepancies between 3 ds hear velocity from either rayleigh vsv o r love vsh waves are examined both in view of non uniqueness of tomographic solutions and radial anisotropy low shear wave speed and radial anisotropy with vsh vsv are most prominent in mid lower crust in area northwest to the lijiang muli fault and around the red river and xiaojiang faults this anisotropy could be caused by sub horizontal mica fabric and its association with low velocity zones suggests mica alignment due to flow in deep crustal zones of relatively low mechanical strength citation huang h h yao andr d vanderhilst 2010 radialanisotropyinthecrust of se tibet and sw china from ambient noise interferometry geophys res lett 37 l21310 doi 10 1029 2010gl044981

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

numerical studies of non newtonian mantle convection abstract numerical model computations have been carried out to determine how the stress dependence of non newtonian viscosity affects the flow structure of thermal convection the viscosity laws have been chosen in accordance with present knowledge of upper mantle rheology based on the diffusion and dislocation creep laws of olivine the results show that there are important differences between the structures of newtonian and non newtonian convection while the newtonian models are insufficient in some respects the non newtonian solutions can explain the characteristics of the real mantle flow however this may require a faster plastic deformation than power law dislocation creep at least in the high stress regions of the mantle e g at the active plate margins

low wave speed zones in the crust beneath se tibet revealed by ambient noise adjoint tomography we present a refined 3d crustal model beneath se tibet from ambient noise adjoint tomography different from ray theory based tomography adjoint tomography in this study incorporates a spectral element method sem and takes empirical green s functions egfs of rayleigh waves from ambient noise interferometry as the direct observation the frequency dependent traveltime misfits between sem synthetic green s functions and egfs are minimized with a preconditioned conjugate gradient method meanwhile the 3d model gets improved iteratively utilizing 3d finite frequency kernels the new model shows 3 6 shear wave speed increasing beneath the western sichuan basin scb depth 15 km and the central chuan dian block cdb and 6 12 shear wave speed reduction in the mid lower crust beneath the northern and the southern cdb the inferred spatial pattern of low wave speed zones consistent with possible partial melt suggests more complex and disconnected geometry than the pervasive narrow zone from the channel flow models

partial melt in the upper middle crust of the northwest himalaya revealed by rayleigh wave dispersion article i nfo seismic shear wave velocities are sensitive to the partial melts that should be present in the himalayan orogen if low viscosity channel flow is active at the present day we analyzed regional earthquakes in the western himalaya and tibet recorded on 16 broadband seismometers deployed across the nw indian himalaya from the indian platform to the karakoram range we used a multiple filter technique to calculate the group velocity dispersion of fundamental mode rayleigh waves and then inverted the dispersion records to obtain separate one dimensional shear wave velocity models for five geologic provinces the tibetan plateau ladakh arc complex indus tsangpo suture zone tethyan himalaya and himalayan thrust belt our velocity models show a low velocity layer lvl with 7 17 velocity reduction centered at 30 km depth and apparently continuous from the tethyan himalaya to the tibetan plateau this lvl shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile of the possible explanations for low velocity and low resistivity in the mid crust only the presence of melts or aqueous fluids or both satisfactorily explains both sets of observations elevated heat flow observed in the nw himalaya implies that if aqueous fluids are present in the mid crust then the mid crust is well above its solidus comparison of our results with laboratory measurements and theoretical models suggests 3 7 melt is present in a channel in the upper middle crust of the nw himalaya at the present day and the physical conditions to enable active channel flow may be present

variable viscosity convection and partial melting in the continental asthenosphere convection beneath continents may be associated with the generation of partially molten zones at depth experimentally determined non newtonian temperature and pressure dependent creep laws for olivine were used to model upper mantle convection in the presence and absence of a melt phase in our models convection is chaotic and exhibits a strong time dependence using model dry and water undersaturated solidus relations partially molten regions develop at asthenospheric depths forming low viscosity regions the existence of these regions is linked to convection and consequently varies in space and time with an episodicity of 5 to 10 m y in good agreement with observations based on seismic tomography and the episodicity and volumetric abundance of surface volcanics for an average continental heat flux of about 53 3 mw m2 maximum melt fractions range between 0 and 2 dry solidus and 2 and 4 water undersaturated solidus on the basis of experimental deformation studies we employed an empirical relationship between stress and grain size to assess the grain size variations possible in the upper mantle the grain sizes obtained millimeters to centimeters are in good agreement with grain sizes observed in natural examples the smallest grain sizes occur along narrow zones of high stress within the deepest part of the lithosphere the largest grain sizes are found in the overlying lithosphere and in partially molten asthenospheric regions as our models scale well to natural observations these results appear to preclude diffusion creep n 1 as a viable upper mantle steady state deformation mechanism

asymmetric delamination and convective removal numerical modeling comparison with evolutionary models for the alboran sea region convective removal and mantle delamination are geodynamical mechanisms proposed to explain the presence of extension in the alboran sea within a regional context of compression using a new thermomechanical algorithm we present here a quantitative evaluation and comparison of conceptual models based on these geodynamical mechanisms in contrast to the in situ convective removal process the laterally propagating delamination mechanism is shown here to be consistent with first order features of the alboran sea such as the thinning thickening distribution intermediate depth seismicity and upper mantle structure imaged by seismic tomography the lower crust is predicted to reach depths of 100 150 km in some areas due to mechanically driven viscous drag of the downwelling mantle

deformation of the core mantle boundary induced by spherical shell compressible convection we derive the single mode mean field equations for spherical shell convection with variable viscosity and compressibility included core mantle boundary cmb topography produced by whole mantle convection is computed for various rheologies and different thermal boundary conditions at cmb results for long wavelength undulations show the extreme sensitivity of these signatures to rheological and equation of state parameters in order to obtain excess ellipticity of about 1 km the activation energy and volume in the lower mantle should be low and the temperatures at cmb around 4 500 k in accord with recent experimental findings

topography and geoid undulations caused by small scale convection beneath continental lithosphere of variable elastic thickness summary the effect of subcontinental upper mantle convection on topography gravity and geoid undulation is studied with special regard to the effective elastic thickness of the continental lithosphere hel we designed a numerical model of the upper mantle lithosphere system consisting of a constant viscosity fluid overlayed by a partly elastic plate the boundary tractions stresses at the top of the convecting mantle model were applied to the lithosphere model for which we used the thin plate approximation the total thickness of the lithosphere was kept constant to 150 km while the elastic thickness was varied between 0 and 150 km the aspect ratio of the convecting mantle was varied between 0 7 and 3 and the rayleigh number between 104 and 106 our calculations demonstrate the effect of a thick elastic lithosphere on the surface observables peak to peak averaged amplitude of topography geoid undulation and gravity are strongly reduced and their spectra are low pass filtered for an aspect ratio less than 1 and a very thick elastic lithosphere topography tends to zero and while geoid undulation and gravity may even become negative above the up welling plume the spectral relation of geoid or gravity to topography admittance is nearly unaffected for hie 70km for aie 70km admittance values for all wavelengths generally decrease with increasing elastic thickness of the plate and may become negative geoid admittance for the basic mode increases with larger aspect ratio while gravity admittance for hie 100 km slightly decreases associating higher rayleigh numbers of the convecting fluid with lower viscosity an increase in rayleigh number leads to considerable decrease of the surface observables however admittance values are only slightly affected and decrease approximately linearly with increasing rayleigh number comparison of our results with observations of the topography spectrum and the free air gravity admittance for north america suggests that large aspect ratio cells beneath continents are unlikely however convection cells of aspect ratio 1 or less underlying an elastic lithosphere of at least 100 km thickness is in agreement with data the rayleigh number should be as high as 106

inverting interpolated receiver functions with surface wave dispersion and gravity application to the western u s and adjacent canada and mexico we use p wave receiver functions from the western u s and adjacent regions to construct a receiver function wavefield interpolation scheme that helps to equalize the lateral sampling of the receiver functions and the surface wave dispersion and to greatly simplify the receiver functions spatial interpolation and smoothing suppress poorly sampled and difficult to interpret back azimuthal variations and allow the extraction of the first order features in the receiver function wavefield including observations from several ray parameter ranges we combine the interpolated receiver functions with rayleigh wave dispersion estimates and surface gravity observations to estimate the 3 d shear wave speed beneath the region speed variations in the 3 d model correlate strongly with expected geologic variations and illuminate broad scale features of the western u s crust and upper mantle the model is smooth self consistent and demonstrates the compatibility of the interpolated receiver functions and dispersion observations

joint inversion of surface wave velocity and gravity observations and its application to central asian basins shear velocity structure 1 we implement and apply a method to the jointly inverted of surface wave group velocities and gravity anomalies observations surface wave dispersion measurements are sensitive to seismic shear wave velocities and the gravity measurements supply constraints on rock density variations our goal is to obtain a self consistent three dimensional shear velocity density model with increased resolution of shallow geologic structures we apply the method to investigate the structure of the crust and upper mantle beneath two large central asian sedimentary basins the tarim and junggar the basins have thick sediment sections that produce substantial regional gravity variations up to several hundred milligals we used gravity observations extracted from the global gravity model derived from the gravity recovery and climate experiment grace satellite mission we combine the gravity anomalies with high resolution surface wave slowness tomographic maps that provide group velocity dispersion values in the period range between 8 and 100 s for a grid of locations across central asia to integrate these data we use a relationship between seismic velocity and density constructed through the combination of two empirical relations one determined by nafe and drake most appropriate for sedimentary rocks and a linear birch s law more applicable to denser rocks the basement an iterative damped least squares inversion including smoothing is used to jointly model both data sets using shear velocity variations as the primary model parameters results show high upper mantle shear velocities beneath the tarim basin and suggest differences in lower crust and upper mantle shear velocities between the eastern and western tarim

geodetic gps measurements in south iceland strain accumulation and partitioning in a propagating ridge system 1 gps observations in south iceland between 1994 and 2003 are compared with two dimensional elastic half space and viscoelastic coupling models for two parallel rift zones representing the western and eastern volcanic zones wvz evz gps data from the hreppar block between the wvz and evz fit a rigid block model within uncertainties spreading rates across the wvz increase from 2 6 0 9 mm yr in the northeast to 7 0 0 4 mm yr in the southwest conversely spreading rates in the evz decrease from 19 0 2 0 mm yr in the northeast to 11 0 0 8 mm yr in the southwest the direction of ridge propagation summed extension rates across the two rift zones are approximately constant and equal to the total plate rate 18 20 mm yr consistent with a simple propagating ridge model whereby the wvz is deactivating in the direction of evz propagation the coupling model confirms results from the simple elastic half space model including relatively shallow locking depths 5 km beneath the rift zones and allows for an estimate of mean viscosity 1019 1020 pa s beneath the elastic layer the location of maximum surface velocity gradient in the evz presumably the locus of subsurface magma accumulation and future rifting does not coincide with the 1783 1784 lakagigar fissure eruption but is 20 km to the west on the bar abunga veidivotn fissure swarm this had a small volume eruption in 1862 1864 but a major eruption in 1477 a d

dynamical effects from equation of state on topographies and geoid anomalies due to internal loading the effects of mantle compressibility on geoid and topographic signatures caused by internal loading in the mantle are studied for a cartesian compressible model the green s functions of these geophysical signatures are determined by solving a two point boundary value problem with depth for each horizontal wave number associated with the density anomaly various types of equations of state have been considered which include both constant and depth dependent thermodynamic parameters surface topographies are not changed too much by the effects of mantle compressibility even for long wavelengths the deformation of bottom boundary is not seriously influenced by mantle compressibility except for long wavelengths and large viscosity contrasts between the upper and lower mantles geoid signals with horizontal wavelengths exceeding 10 000 km can be greatly influenced by mantle compressibility for large viscosity contrasts differences of up to 100 between incompressible and compressible geoid responses can be found the admittance function also shows great sensitivity to mantle compressibility at long wavelengths there are significant differences in the geophysical signatures from the ways in parameterizing the viscosity stratification in the mantle an exponentially dependent viscosity model produces smaller geoid and surface topographical signals than for a single step function viscosity model

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

a simple model of whole mantle convection isotopic evidence of the chemical variability of basalts implies the existence of distinct long lived chemical reservoirs in the mantle this has commonly been interpreted as evidence that mantle convection is layered since known models of whole mantle convection have too much shearing to allow preservation of these reservoirs however the geochemical evidence requires at least three reservoirs and models of layered convection have dynamical difficulties casting serious doubt on the viability of these models this difficulty has been resolved in the present paper by the development of a simple model of whole mantle convection which allows the preservation of chemically distinct reservoirs in this model heating of the mantle from below produces axisymmetric plumes which carry heat upward through the mantle without causing overturning hence the lower mantle is much less sheared than in the usual two dimensional model mantle convection is driven by the penetration of cold lithospheric slabs from above it is argued that the pattern of convection is governed primarily by the dependence of viscosity on temperature and this dependence drives the mantle toward an isoviscous state these ideas are quantified in a simple one dimensional model of mantle convection in which the slab mass flux as a function of radius is governed by the requirement that the temperature profile remains isoviscous the resulting convection cycles material through the upper mantle more rapidly than through the lower mantle this provides a mechanism for an initially homogeneous mantle to become chemically stratified with a depleted upper mantle and primitive lower mantle

dynamical and thermal structure of a subduction zone influence of slab geometry on the convective state of the earth s upper mantle preliminary results received 29 march 1996 accepted 3 july 1996 abstract the aim of this study is to investigate the influences of plate kinematics and of the downgoing slab s geometry on internal dynamics and thermal structure in the vicinity of a subduction zone a systematic study depending on these parameters has been carried out based on a two dimensional finite differences code in a convective domain representing the upper mantle plate displacements are prescribed by kinematic boundary conditions in particular the subducting plate moves and dips at a constant velocity under the slower overriding one typical subduction dips 30 45 and 70 have been studied for a realistic rayleigh number of about l05 and for plate velocities around a coupling velocity of 2 3 cm year 1 this limit value was inferred from the balance between surface forces and buoyancy forces for any subduction dip and plate velocities lower than this coupling velocity plate kinematics variations induce significant effects on thermal and mineralogical patterns at greater plate velocities these patterns stabilize for plate velocities of the order of the coupling velocity a subduction dip decrease of 40 involves a heating of the downgoing plate of 100degrees and a depth increase of 20 km of the olivine spinel phase transition in this plate in the particular case of a non moving overriding plate the balance between this plate and underlying internal dynamics requires a moving plate velocity lower than 2 3 cm year j for a lower dip moreover a shallow subduction enhances a lateral large scale convection under the upper plate even if this plate is motionless in contrast to the case of a steep subduction this is consistent with observed back arc extension at the surface which is often associated with steep subductions our model suggests a spreading zone of between 500 and 1000km in the upper plate for a subduction velocity lower than 2 3 cm year i and a rayleigh number of about l05 1 introduction the earth s mantle undergoes a rayleigh benard thermal convection coupled to oceanic lithospheric plate movements rigid and moving lithospheric corresponding author plates form the thermal and mechanical limit bound aries of the earth s mantle convection cells one of the concerns of numerical modelling is to account for a real dynamic convection model coupled to plate tectonics to force the movement of rigid plates one may either model a dynamic plate by prescribing a lower viscosity at plate margins or impose the horizontal surface velocity davies 1989 0031 9201 97 17 00 copyright 1997 elsevier science b v all rights reserved ph s0031 9201 96 03209 8

geologic geochemical and geophysical consequences of plume involvement in the emeishan flood basalt province prevolcanic kilometer scale lithospheric doming in the emeishan large igneous province southwest china allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography geology geochemistry and geophysics of the region systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments the extent of erosion thickness and chemistry of volcanic rocks and the crust mantle structure these features which are best explained by a mantle plume may be used to track older plume sites in the geologic record

high resolution crustal model of se tibet from joint inversion of seismic p wave travel times and bouguer gravity anomalies and its implication for the crustal channel flow abstract we present a high resolution 3 d crustal model of southeastern tibetan plateau se tibet by joint inversion of local earthquake p wave travel times and bouguer gravity anomalies the joint inversion improves the resolution compared with travel time only tomography and more clearly depicts the spatial distribution of low velocity zones in the crust our joint inversion model finds no evidence for crustal channel flow in this part of se tibet however our velocity model shows three distinct low velocity zones in the mid crust which are generally associated with the major fault systems that control the deformation of se tibet in the cenozoic the thick mafic ultramafic underplating beneath the emeishan large igneous province effectively blocks the crustal flow from tibetan plateau and the crustal low velocity zone beneath the luzhijiang fault and xiaojiang fault is more likely due to localized asthenospheric upwelling at the southern end of nujiang fault the joint inversion model reveals the magmatic system beneath the tengchong volcano that facilitates melt migration from uppermost mantle to the magma chambers in the upper to mid crust

measurement of the melting curve of mg0 9fe0 1sio3 at lower mantle conditions and its geophysical implications an experimental determination of the melting of mg0 9fe0 1sio3 under lower mantle conditions is presented samples were compressed and laser heated nd yag in a mao bell type diamond cell the temperature fields generated in the sample are measured using a simple tomographic technique and spectroradiometer mg0 9fe0 1sio3 in the perovskite structure melts at 3000 300 k independently of pressure over the range of 22 65 gpa the melting curve of mg0 9fe0 1sio3 represents an experimental upper bound upon the geotherm from 700 to 1500 km this melting curve corresponds to a homologous temperature of 0 70 or 0 85 for one or two layer mantle convection respectively this is consistent with either one or two layer mantle convection and a lower mantle of constant viscosity based upon homologous temperature arguments

hypocenter velocity structure coupling in local earthquake tomography abstract local earthquake tomography involves the use of seismic wave arrival times from earthquake sources with uncertain locations for the inference of three dimensional variations in seismic velocity formally this inverse problem contains explicit coupling between the earthquake hypocenters and the velocity structure however many studies fail to treat the hypocenter velocity structure coupling explicitly an examination of previous studies provides evidence that this failure can lead to significant bias in the resulting velocity model a set of simple hypothetical test cases clearly demonstrates the seriousness of the potential bias when coupling is neglected producing models that underestimate the actual velocity perturbations by about a factor of four under the same conditions strict simultaneous inversion and simultaneous inversion with parameter separation both produce accurate inverted models the use of appropriate station corrections and iteration of the solution can improve the results when the coupling is ignored but velocity anomalies will still be underestimated by about a factor of two at best

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

a benchmark comparison for mantle convection codes summary we have carried out a comparison study for a set of benchmark problems which are relevant for convection in the earth s mantle the cases comprise steady isoviscous convection variable viscosity convection and time dependent convection with internal heating we compare nusselt numbers velocity temperature heat flow topography and geoid data among the applied codes are finite difference finite element and spectral methods in a synthesis we give best estimates of the true solutions and ranges of uncertainty we recommend these data for the validation of convection codes in the future

compressible thermochemical convection and application to lower mantle structures a new finite element code for compressible thermochemical convection is developed to study the stability of a chemical layer at the base of the mantle using composition dependent compressibility and a density difference between compositions at a reference pressure a composition dependent density profile is derived together with depth dependent thermal expansion this combination of parameters yields a wide range of dynamic evolutions for the chemical layer the chemical structures are classified into five major categories classical plumes mushroom shaped plume domes ridges and continuous layers and a few abnormal cases such as hourglass shaped plumes and columnar plumes several models have a chemical structure morphologically similar to the african low v s structure in the lower mantle at least at a single time guided by our models several dynamic scenarios are proposed for the dynamic nature of the lower mantle low velocity structures a k a superplumes including plumes at an early stage plume clusters ridges passive piles sluggish domes and high bulk modulus domes we predict seismic velocity anomalies from these dynamic models the thermoelastic parameters used in the conversion are additional constraints we compare the density structure with normal mode inversion the predicted seismic signature observations and the required thermoelastic parameters with mineral physics data among the proposed scenarios only the scenario of high bulk modulus domes satisfies all constraints simultaneously the implication on the geochemistry and mineralogy of lower mantle chemical structures is discussed

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

the dynamical and thermal structure of deep mantle plumes abstract if the interpretation of the d layer at the base of the mantle as a thermal boundary layer with a temperature increment in the order of 800 k is correct then the formation of deep mantle plumes to vent material from it appears inevitable we demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long lived chimneys that are 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material the speed of flow up the axis of the plume is correspondingly fast 1 6 m y 1 at the base and 4 8 m y 1 at 670 km depth thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there this convergence which contributes only a small increment to the flux of material up the plume also serves to throttle the flow in the chimney the global plume mass flux necessary to transport 1 6 10 12 w of core heat upward through the mantle is 1 8 10 6 kg s 1 at its base plume material is probably still significantly below its solidus or eutectic temperature but substantial partial melting is very likely as it rises we speculate that a small fraction of this fluid component eventually emerges at the surface in hot spots with the fate of the remainder being unknown the behaviour and properties of d and of plumes are closely coupled not only are plumes a necessary consequence of a thermal boundary layer but their existence is impossible without that layer

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

heterogeneity and anisotropy of the lithosphere of se tibet from surface wave array tomography 1 understanding the geotectonic evolution of the southeastern tibetan plateau requires knowledge about the structure of the lithosphere using data from 77 broadband stations in sw china we invert rayleigh wave phase velocity dispersion curves from ambient noise interferometry t 1 0 40 s and teleseismic surface waves t 2 0 150 s for 3 d heterogeneity and azimuthal anisotropy in the lithosphere to 150 km depth our surface wave array tomography reveals 1 deep crustal zones of anomalously low shear wave speed and 2 substantial variations with depth of the pattern of azimuthal anisotropy upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern himalayan syntaxis with fast directions generally parallel to the main strike slip faults the mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south the tomographically inferred 3 d variation in azimuthal anisotropy helps constrain the source region of shear wave splitting south of 26 n off the high plateau most of the observed splitting can be accounted for by upper mantle anisotropy but for stations on the plateau proper with thick crust crustal anisotropy cannot be ignored on long wavelengths the pattern of azimuthal anisotropy in the crust differs from that in the mantle this is easiest explained if deformation varies with depth the deep crustal zones of low shear wave speed and presumably mechanical strength may represent loci of ductile deformation but their lateral variation suggests that in se tibet localized crustal channel flow and motion along the major strike slip faults are both important

deep and near surface consequences of root removal by asymmetric continental delamination abstract the occurrence of continental delamination has been proposed for a number of areas characterized by highly variable geodynamic settings in this study we present results of numerical simulations considering different initial setups representative for geodynamic scenarios where delamination could potentially develop to mimic a post collisional orogenic scenario we have designed an initial state characterized by the presence of an area of orogenic lithosphere with both crustal and lithospheric roots in a second setup we have considered a lithospheric root representative of a remnant slab with a flat overlying crust we focus on predicted evolution of surface and near surface observables namely the crustal structure surface heat flow and isostatic and dynamic topography evolution our results show that a high density orogenic lower crust likely related to the presence of eclogite significantly accelerates the sinking of the lithospheric mantle the pattern of local isostatic elevation is characterized by laterally migrating surface uplift subsidence this pattern is shown to be little sensitive to lower crust density variations in contrast predicted dynamic topography is more sensitive to these changes and shows surface subsidence adjacent to the delaminating lithospheric mantle for the model with a high density lower crust and surface uplift above the slab for a model with a less dense lower crust the reason for uplift in this second model is that the effect of the positive buoyancy of the thickened crust overwhelms the effect of negative buoyancy of the slowly sinking lithospheric mantle we infer from our modeling that there is not a specific characteristic pattern of topography changes associated with delamination but it depends on the interplay between highly variable factors as slab sinking velocity asthenospheric upwelling and changes in crustal thickness

a melting model for the lowermost mantle using clapeyron slopes derived from experimental data consequences for the thickness of ultralow velocity zones ulvzs 1 ultralow velocity zones ulvzs are relatively thin regions directly above the core mantle boundary cmb that exhibit marked seismic p and s velocity reductions a viable explanation for the reduction is the presence of melt fractions within ulvzs since melt was found to be denser than solid in melting experiments at lowermost mantle pressures partially molten ulvzs should exhibit negative buoyancy using published experimental data we present a melting model based on clapeyron slopes for the formation of ulvzs as partially molten regions above the cmb and apply the resulting melting curves and latent heat effects in fully dynamic regionally constant viscosity convection simulations of the lowermost mantle we find that the height of the ulvzs depends only moderately on rayleigh number but strongly decreases with increasing excess density of melt over solid the models predict excess density of at least 1 to explain observed heights the combined effect of topography and latent heat of melting reduces the vigor of mantle convection only very slightly while if combined with a decrease of the ulvz viscosity mantle flow velocities are significantly enhanced near the cmb and overall mantle temperatures are notably increased ulvz heights are found to be insensitive to ulvz viscosity for the range isoviscous to 1 100 the viscosity of the ambient mantle

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

magmatic underplating and crustal growth in the emeishan large igneous province sw china revealed by a passive seismic experiment abstract in an attempt to characterize the subsurface structure that is related to fossil mantle plume activity a comprehensive geophysical investigation was conducted in the emeishan large igneous province elip the nature and geometry of the crust were examined within the scheme of the domal structure of elip which comprises the inner intermediate and outer zones which are defined on the basis of the biostratigraphy of pre volcanic sediments the bulk crustal properties within the inner zone are characterized by high density high p wave velocity high vp vs ratios and large crustal thickness a visible continuous seismic converter is present in the upper part of the crust in the whole intermediate zone and the eastern part of the inner zone but it is absent in the inner zone where another seismic converter is observed in the lower part of the crust the geometric configuration of these converters is attributable to the addition of mantle derived melts to the pre existing crust and subsequent interaction between them the crustal geometry which is delineated by the migrated image of receiver functions from the passive seismic experiment and the crustal properties collectively suggest that a mafic layer of 15 20 km thickness and 150 180 km width exists at the base of the crust in the inner zone such a mafic layer reflects a vertical crustal growth through magmatic underplating at the base of the crust and intraplating within the upper crust the salient spatial correlation between the deep crustal structure and the dome strongly supports a genetic link between crustal thickening and plume activity if the pre volcanic domal uplift is generated by the permian emeishan mantle plume this arrangement is further supported by the consistency of the extent of crustal uplift estimated by isostatic equilibrium modeling and sedimentary data this study therefore characterizes and provides evidence for a plume modified crust in a large igneous province

collision of the ganges brahmaputra delta with the burma arc implications for earthquake hazard abstract we take a fresh look at the topography structure and seismicity of the ganges brahmaputra delta gbd burma arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential the gbd the world s largest delta has been built from sediments eroded from the himalayan collision these sediments prograded the continental margin of the indian subcontinent by 400 km forming a huge sediment pile that is now entering the burma arc subduction zone subduction of oceanic lithosphere with 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land the prism starts at an apex south of the gbd shelf edge at 18 n and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit the front of the prism is blind buried by the gbd sediments thus the deformation front extends 100 km west of the surface fold belt beneath the comilla tract which is uplifted by 3 4 m relative to the delta this accretionary prism has the lowest surface slope of any active subduction zone the gradient of the prism is only 0 1 rising to 0 5 in the forearc region to the east this low slope is consistent with the high level of overpressure found in the subsurface and indicates a very weak detachment since its onset the collision of the gbd and burma arc has expanded westward at 2 cm yr and propagated southwards at 5 cm yr seismic hazard in the gbd is largely unknown intermediate size earthquakes are associated with surface ruptures and fold growth in the external part of the prism however the possibility of large subduction ruptures has not been accounted for and may be higher than generally believed although sediment clogged systems are thought to not be able to sustain the stresses and strain weakening behavior required for great earthquakes some of the largest known earthquakes have occurred in heavily sedimented subduction zones a large earthquake in 1762 ruptured 250 km of the southern part of the gbd suggesting large earthquakes are possible there a large but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism and is the only known candidate for a rupture of the plate boundary along the subaerial part of the gbd burma arc collision zone

the effects of adiabatic and viscous heatings on plumes we have assessed the influence of viscous and adiabatic heatings on the transient development of plumes by employing the time dependent convection equations in the extended boussinesq limit both temperature dependent and temperature and pressure dependent viscosities are employed for constant thermal expansion of coefficient we find the vigor of plume flows is diminished with larger magnitudes of the thermal expansivity this effects is caused by the local domination of adiabatic decompression over viscous heating inside the developing plume for both the cartesian and axisymmetric domains for deep mantle flows a thermal expansivity decreasing with depth is required for thin plume structures to exist values of the thermal expansivity contrast which can promote plume growth are in accord with estimates from recent studies in seismic equation of state copyright american geophysical union 1987

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

present seismic evidence for a boundary layer at the base of the mantle to improve the global coverage needed in investigating the observational consequences of boundary layer models the existing data set has been extended with long period diffracted p and sh from the sro network and short period diffracted p from the norsar array it was found that on average the amplitude decay of diffracted sh is relatively small in accordance with the majority of previously published wwssn data and consistent with the observed weakness in dispersion of this phase these results in combination with the apparent absence of anomalously high dt d can be explained by a relatively thin 100 km low velocity zone at the base of the mantle long period diffracted p wave data probably lack the required resolution for detecting a relatively thin transition zone such a zone with relatively low velocity is allowed by an average of travel time and amplitude data across the core shadow boundary and appears to be required by the norsar observations of short period diffraction far into the shadow however recent results of individual short period p wave studies pertaining to geographically limited regions only including also the norsar observations are partly conflicting unless lateral variations exist on balance current thermal models appear to be consistent with but not required by the present seismic evidence

on the effects of the lithosphere on mantle convection and evolution abstract an idealized model is discussed for plate motion and mantle convection it consists of a numerical square box filled with a newtonian boussinesq fluid heated from below at the surface the fluid has a high viscosity i e it contains a stiff lid which may or may not be laterally decoupled by weakness zones lid stiffness and coupling were varied systematically only if the lid is effectively decoupled laterally does it move as a quasi rigid plate it is then driven by the lateral pressure differential from the rising and descending plumes in the fluid and it moves against viscous shear at its bottom gravitational sliding and sinking may also drive the plate if allowed for in the model a decoupled freely moving stiff plate effectively removes heat from the interior a strongly coupled stiff lid on the other hand or one with a very large aspect ratio inhibits heat transfer through the convecting layer if heating occurs by a given flux through the bottom or internal heat generation the internal cell temperatures become very high because the stagnant surface lid can only conduct the heat by assuming the required temperature gradient throughout its thickness since the earth is partly covered by conducting continental lids which in addition contain much of the heat sources the thermal history may be affected caution must be exercised in parameterized convection modelling in this respect the overall effect cannot easily be predicted because mantle flow and continental plates will interact the importance of mechanical decoupling for plate motion strongly calls for thorough studies of the decoupling mechanisms particularly at convergent plate boundaries bending stick slip processes and the role of volatiles

lateral migration of a foundering high density root insights from numerical modeling applied to the southern sierra nevada abstract the southern sierra nevada is a geodynamically complex region where several models have been proposed to explain the rapid removal of lithospheric mantle occurring sometime between 8 0 and 3 5 ma tomographic studies show the presence of an east dipping slab shaped fast seismic anomaly reaching to about 300 km depth below the western sierras and great valley and receiver function studies indicate a broad region of lithosphere removal this work presents thermo mechanical modeling of asymmetric foundering of a high density batholithic root with lateral intrusion of asthenospheric material the predicted evolution is controlled by a the upwelling of buoyant asthenosphere facilitated by the presence of a weakened lithospheric mantle adjacent to a dense batholitic root b the westward inflow enabled by a low viscosity lower crust and c negative buoyancy of a batholithic dense root the dynamics of these models can be characterized as a slowly migrating lithosphere foundering process driven by the density anomaly of the ultramafic root but controlled by the magnitude of the lower crustal viscosity which determines the rate at which asthenospheric material can flow into the opening lower crustal gap final model predicted upper mantle structure is compatible with existing tomographic images and the observed v shape geometry of the moho below the western margin of the southern sierra nevada model predicted topography is also generally consistent with observations and shows a monotonous uplift of the high region since 7 ma and presently ongoing and an area of maximum subsidence west of the area of the v shaped moho due to the pull exerted by the sinking of the high density root

water induced convection in the earth s mantle transition zone 1 water enters the earth s mantle by subduction of oceanic lithosphere most of this water immediately returns to the atmosphere through arc volcanism but a part of it is expected as deep as the mantle transition zone 410 660 km depth there slabs can be deflected and linger before sinking into the lower mantle because it lowers the density and viscosity of the transition zone minerals i e wadsleyite and ringwoodite water is likely to affect the dynamics of the transition zone mantle overlying stagnant slabs the consequences of water exchange between a floating slab and the transition zone are investigated in particular we focus on the possible onset of small scale convection despite the adverse thermal gradient i e mantle is cooled from below by the slab the competition between thermal and hydrous effects on the density and thus on the convective stability of the top layer of the slab is examined numerically including water dependent density and viscosity and temperature dependent water solubility for plausible initial water content in a slab 0 5 wt an episode of convection is likely to occur after a relatively short time delay 5 20 ma after the slab enters the transition zone however water induced rheological weakening is seen to be a controlling parameter for the onset time of convection moreover small scale convection above a stagnant slab greatly enhances the rate of slab dehydration small scale convection also facilitates heating of the slab which in itself may prolong the residence time of the slab in the transition zone

insight into the origin of the tengchong intraplate volcano and seismotectonics in southwest china from local and teleseismic data received 18 june 2008 revised 22 february 2009 accepted 10 march 2009 published 13 may 2009 1 a high resolution tomographic image of the crust and upper mantle under yunnan province in southwest china was determined by using a large number of teleseismic data measured precisely from digital seismograms as well as local earthquake arrival times recorded by the dense yunnan seismic network the grid spacing of the tomographic model is 70 km 78 km in the horizontal directions and 4 70 km in depth our resulting model shows a clear low velocity low v 0 3 with respect to the average one dimensional velocity model column extending from the surface down to about 400 km depth under the active tengchong volcano and some high velocity high v anomalies existing in the mantle transition zone furthermore the low vanomaly extends horizontally toward the northeast at 250 to 400 km depth combining our tomographic results with geochemical and geological investigations we infer that the tengchong volcano is a rift related volcano caused by the subduction and dehydration of the indian plate as well as corner flow in the mantle wedge though the subducted slab is a continental plate our results show that the upwelling flow under tengchong originates at 400 km depth a prominent low v anomaly along the red river fault zone extends down to the upper mantle 100 km depth reflecting that the red river fault zone may have cut through the crust to the upper mantle the two dayao earthquakes m 6 1 and m 6 2 in 2003 occurred on the margin of high v anomalies and are underlain by a prominent low v anomaly in the lower crust and upper mantle suggesting that the dayao earthquakes may be associated with fluids released from the dehydration of the subducted indian slab these results are significantly improved compared with previous studies and provide new seismic constraints on the dynamic processes of the india asia collision

seismicity and structure of akutan and makushin volcanoes alaska using joint body and surface wave tomography joint inversions of seismic data recover models that simultaneously fit multiple constraints while playing upon the strengths of each data type here we jointly invert 14 years of local earthquake body wave arrival times from the alaska volcano observatory catalog and rayleigh wave dispersion curves based upon ambient noise measurements for local vp vs and hypocentral locations at akutan and makushin volcanoes using a new joint inversion algorithm the velocity structure and relocated seismicity of both volcanoes are significantly more complex than many other volcanoes studied using similar techniques seismicity is distributed among several areas beneath or beyond the flanks of both volcanoes illuminating a variety of volcanic and tectonic features the velocity structures of the two volcanoes are exemplified by the presence of narrow high vp features in the near surface indicating likely current or remnant pathways of magma to the surface a single broad low vp region beneath each volcano is slightly offset from each summit and centered at approximately 7 km depth indicating a potential magma chamber where magma is stored over longer time periods differing recovery capabilities of the vp and vs data sets indicate that the results of these types of joint inversions must be interpreted carefully

the role of shear heating in lubricating mantle flow abstract the role of viscous heating in mantle flows has been studied within the framework of a steady state two dimensional cartesian model with a composite non newtonian and newtonian rheology there is temperature dependence in the upper mantle rheology and temperature and pressure dependent rheology in the lower mantle we have considered adiabatic and viscous heating using the extended boussinesq approximation we have selected as the only control variable the temperature at the core mantle boundary cmb we have compared systematically both the boussinesq and extended boussinesq solutions for the same rheology and same amount of internal heating by varying the temperature at the cmb from 2000 k to around 10 000 k viscosity contrasts in the system increase with the vigor of convection and reach up to five orders in magnitude both the vigor of convection and the global viscous heating increases systematically with the temperature at the cmb the rms velocity shows a power law type dependence on the temperature at the cmb with a power law exponent of 1 5 for high cmb temperatures greater then 3500 k a focused concentration of shear heating promotes the development of high strain rate and low viscosity zones next to descending flows which is primarily non newtonian in character in the high cmb temperature regime much more pronounced shear zones are developed along the downwelling for models with viscous heating present than for the boussinesq flows illustrating the important role played by mechanical heating on lubricating limbs in mantle circulation

shear heating in continental strike slip shear zones model and field examples summary a two layer crust and upper mantle finite diverence steady state thermomechanical model of a long lived several million years lithospheric strike slip fault is presented and its predictions compared with field observations from various major fault zones in order to estimate the maximum amount of shear heating all mechanical energy is assumed to be dissipated in heat in ductile as well as in brittle layers deformation follows a friction law in the brittle layer s and a power flow law in the ductile one s variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored shear heating is found to be more important in fault zones avecting an initially cold lithosphere and increases with slip rate friction coeycient and stivness of materials in extreme cases slip rate of 10 cm yr 1 stiv lithosphere shear heating could lead to temperature increases close to 590 c at the moho and 475 c at 20 km depth for more common cases shear heating leads to smaller temperature increases but can still explain high grade metamorphic conditions encountered in strike slip shear zones however modelled temperature conditions often fall short of those observed this could be due to heat transport by mechanisms more eycient than conduction common syntectonic emplacement of granitic melts in ductile strike slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle besides slip rate the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust for hard upper mantle and highly fertile lower crust partial melting could occur at rates of 1 cm yr 1 while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr 1 as a result of shear heating partial melting of the upper mantle could occur in the presence of small amounts of fluids rise of magmas and or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone in nature shear heating would inevitably cause strain localization in the deeper parts of strike slip faults as is often observed in the field for crustal shear zones