extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

flexure of the india plate underneath the bhutan himalaya 1 we investigate flexural geometry and rheology of the india plate beneath the eastern himalaya from a new gravity data set acquired in bhutan compared to the well studied nepal himalaya the obtained bouguer anomaly profiles across the range show shorter wavelength flexure of the lithosphere with a narrower and shallower foreland basin this new data set and seismic moho constraints are interpreted together in terms of lithospheric flexure using a 2 d thermomechanical model our results suggest that the strongest layer of the continental lithosphere beneath bhutan is the upper mantle as it is beneath nepal the observed west to east decrease in flexural wavelength is associated with weakening mantle rheology the simulations show that this decrease can be related to ductile mantle behavior either hydrated wet dunite or dry and hot olivine rheology both models display decoupled lithospheric layers leading to an eastward decrease of flexural rigidity from 10 24 to 5 10 22 n m in nepal and bhutan respectively

deep mantle forces and the uplift of the colorado plateau 1 we introduce a quantitative model of global mantle convection that reconstructs the detailed motion of a warm mantle upwelling over the last 30 ma towards the interior of the southwestern usa from observed present day mantle heterogeneity the onset and evolution of uplift in the central basin and range province and colorado plateau during this time is determined by tracking the topographic swell due to this mantle upwelling we show that 1 the extension and basaltic volcanism post 25 ma in the central basin and range coincides with the arrival and eastward progression of this upwelling and 2 dynamic uplift of the southern colorado plateau totaling about 1 km transpired in the last 20 ma since 10 ma the center of uplift continued northeastward from the southwestern rim of the plateau consistent with a young grand canyon model and eastward sweep of magmatism in the western colorado plateau

temporal variability in currents and the benthic boundary layer at an abyssal station off central california abstract current meter records from seven sequential moorings were collected during 1993 1996 at an abyssal station off central california sta m 4100 m depth the entire 2 1 4 yr time series of the flow at 600 50 and 2 5 m above bottom above near the top of and within the benthic boundary layer were analyzed for mean flow statistics and subtidal and tidal variability the mean current vectors at each altitude were southward at profiles of light transmission and potential temperature collected intermittently during the 2 1 4 yr period showed that the benthic mixed layer at sta m extended on average 40 m above bottom range 15 80 mab evidence for local resuspension of recently deposited detritus came from time lapse photographs of the sea floor that showed a period of near bottom turbidity that corresponded to a period of high near bottom flow however rough estimates of friction velocity indicated that the bed stress usually was too low at sta m to cause local resuspension of sediments we hypothesize that the observed benthic mixed layers contained suspended particles advected from more energetic areas to the north of sta m

resuspension measured with sediment traps in a high energy environment the near bottom sedimentation rates were measured by placing cylindrical sediment traps 10 m above the sea floor on each of six moorings deployed between 4100 and 5100 m along a transect across an energetic deep sea current in the hebble area centered at 40 n 63 w on the nova scotian rise sedimentation rates above the sea floor were monitored with additional traps at 23 54 100 200 and 500 m above the bottom mab on the mooring at 4950 m the total flux at 500 mab for the two week period consisting mostly of primary particles from surface water was 166 mg m2 day and increased exponentially towards the bottom the total flux at 10 mab increased down slope from 1160 mg m2 day at 4158 m where the mean current speed was 8 cm s to a maximum of 77 300 mg m2 day at 5022 m where the mean current speed was 32 cm s then decreased to 59 400 mg m2 day at the mooring at 5076 m the size frequency distributions of large discrete particles such as foraminifera diatoms radiolarians and fecal pellets were quantified in all trap samples to examine whether the large variation in fluxes was due to artifacts such as current velocity or trap tilt based on the source persistence and distribution of these particles we conclude that the large variations in fluxes across the rise and with distance from the sea floor are due primarily to resuspension and resettling of bottom sediments with tilt and current effects on trapping having only a secondary effect the vertical gradients of large particle fluxes suggest effective vertical eddy diffusivities of 102 104 cm2 s using a two dimensional model horizontal advection and secondary circulation probably play a large role in moving large rapidly falling up to 1 cm s particles to a height of 50 100 m above the sea floor

el chichon volcanic ash in the stratosphere particle abundances and size distributions after the 1982 eruption volcanic ash particles collected from the stratosphere after the march april 1982 explosive eruption of el chichon volcano mexico were mostly 2 40 m vesicular shards of silicic volcanic glass that varied in abundance at 16 8 19 2 km altitude from 200 m 3 30 49 n lat in may to 1 3 m 3 45 75 n in october at the minimum the ash cloud covered latitudes 10 60 n in july and 10 s 75 n in october in may and july ash particles were mostly free individual shards and clusters of shards but by october were intimately associated with liquid droplets presumably sulfuric acid in may 1982 the total stratospheric burden of ash was at least 240 tons 2 2 108 g although the total ash injected into the stratosphere by the eruption was probably 480 8400 tons

large scale crustal heterogeneities and lithospheric strength in cratons abstract the rheology and thermal structure of the continental lithosphere are intimately linked in old cratons the effective elastic thickness of the lithosphere has been estimated by various spectral inverse methods based on the correlation between topography and gravity anomalies estimates vary within a very large range from 40 km to 120 km depending on the method used in this paper we use forward models to account for lateral variations in mechanical properties and their effect on the equivalent elastic thickness eet of the lithosphere from these models which allow brittle elastic ductile rheologies and mechanical discontinuities faults we have calculated the strain stress distributions and displacement fields vertical integration of the stress permits a local determination of the effective elastic thickness the computed displacements were used to calculate related bouguer and free air gravity anomalies and compare them with the observations the analysis is applied to the 2000 ma kapuskasing uplift in the superior province of the canadian shield where the presence of a high density block in the upper crust is due to the upthrusting of midcrustal rocks along a major thrust fault the study shows that the stability of this structure on geologic time scales requires a strong lower crustal rheology a cold geotherm and the fault to be healed this study also shows that because of stress dependence of the non linear rheology crustal heterogeneities may cause significant 40 local reductions of the lithospheric strength away from the kapuskasing structure the average strength of the lithosphere remains high eet 100 km conventional methods for estimating the elastic thickness would not resolve such local strength reductions in cratons but would predict depending on the method used highly overestimated or instead underestimated eet

localization of the gravity field and the signature of glacial rebound the negative free air gravity anomaly centred on hudson bay canada shows a remarkable correlation with the location of the laurentide ice sheet suggesting that this gravity anomaly is the result of incomplete post glacial rebound this region however is also underlain by higher than average mantle seismic velocities suggesting that the gravity low might result instead from dynamic topography associated with convective downwellings here we analyse the global gravity field as a simultaneous function of geographic location and spectral content we find that the hudson bay gravity low is unique with anomalously high amplitude in the spectral band where the power from the laurentide ice load is greatest and the relaxation times predicted for viable models of viscous relaxation are longest we estimate that about half of the hudson bay gravity anomaly is the result of incomplete post glacial rebound and derive a mantle viscosity model that explains both this gravity signature and the characteristic uplift rates for the central laurentide and fennoscandian regions this model has a jump in viscosity at 670 km depth comparable to that in dynamic models of the geoid highs over subducted slabs but lacks a low viscosity asthenosphere consistent with a higher viscosity in the upper mantle beneath shields than in oceanic regions

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

mid latitude mesoscale variability neither homogeneous quasi geostrophic turbulence nor the effect of eddies on the interior ocean circulation remains the major focus of mesoscale research as was the case for much of the preceding 20 years following identification of the phenomena concentration on basin wide geographical exploration of eddy properties was rather well established by 1978 over the last few years a promising zero order description and rationalization of the mesoscale field has become available that is eddy energy is largest near strong flows intensification not only in western boundary currents but along their mid latitude open ocean extension roughly as indicated by gyre scale numerical models where eddies develop as a result of instability processes in the mid latitude jet and recirculation regimes and then propagate into the ocean interior our point of view is that mesoscale eddies drive recirculations but do not play a crucial role in the dynamics of the mean flow in the ocean interior in the north atlantic for example the idea is that the downstream increase in transport of the gulf stream is eddy driven but not the flow through the florida straits in addition the fluctuating response to forcing by variable winds may now be widely perceived as forming the horizontally homogeneous background signal in subtropical gyres the time dependent field associated with thermohaline flows at abyssal depths also has a special character

observations and simulation of a bottom ekman layer on a continental shelf a numerical model was used to simulate the bottom ekman layer of a continental shelf region the basis for the model was the mellor and yamada level 2 turbulence closure scheme conservation equations for momentum turbulent kinetic energy and turbulent length scale were utilized in the model the model was used to simulate data taken from a cyclesonde mooring on the peruvian continental shelf in may 1976 as part of the joint ii coastal upwelling ecosystems analysis program the cyclesonde provided mean horizontal velocity temperature salinity and pressure data an intense poleward undercurrent drove the bottom flow regime the most striking feature of the data was the clockwise ekman veering of velocity vectors as the bottom was approached a 48 hour period was chosen for the model simulations the vertical profile of speed 48 hours mean simulated by the model fell within the error bars of the data the corresponding ekman spiral display of the model results also showed good agreement with the observati ons

gravity anomalies flexure and the elastic thickness structure of the india eurasia collisional system wehaveusedbouguergravityanomalyandtopographydatatodeterminetheequivalentelasticthicknessofthelithosphere te in the region of the india eurasia collisional system comparison of observed and modelled gravity anomalies along 1dimensional profiles suggest there are significant variations in te along strike of the himalaya foreland estimates decrease from a high of 70 km in the central region to 30 50 km in the east and west we have verified these inferences of spatial variations using a 2 dimensional non spectral interative flexure and gravity anomaly modelling technique the himalaya foreland forms a high te 40bteb100 km rigid block with a well defined edge as shown by the localisation of faulting and deformation along its northern margin other highteblocks occur to the north beneath the qaidam and sichuan basins the tibetan plateau forms a low te 0bteb20 km weak region that extends from the central part of the plateau into south western china tectonic styles in the india eurasia collisional system therefore involve both drigidt and dnon rigidt blocks where high te rigid blocks are present the stylesdominatedbyunderthrustingofthemorerigidblock wherethecollisionalzoneisnotconstrainedbyrigidblocks however the style appears to be dominated by lower crustal flow and a more continuous style of deformation d 2005 elsevier b v all rights reserved

constraints on the venusian crustal thickness variations in the isostatic stagnant lid approximation 1 the isostatic stagnant lid isl approximation is applied to the long wavelength venusian topography and geoid to place constraints on the thickness of the crust and lithosphere previous studies showed that a purely thermal isostasy model of the lithosphere the stagnant lid accounts for a significant portion of the topography and geoid anomalies here we assume that the remaining discrepancy between the observed and model geoid is due to crustal thickness variations and that the crust the lithosphere and the mantle are in a state of double isostatic equilibrium the lithosphere is isostatically floating on top of the mantle and the crust is isostatically floating on top of the lithosphere the addition of another variable the crustal thickness allows an exact fit to the observed topography and geoid this also makes the solution non unique an exact fit to topography and geoid can be achieved in a broad range of average lithospheric and crustal thicknesses additional constraints such as the gabbro eclogite phase transition and the existence of relatively young volcanism limit this range to some extent the inferred variations in the crustal thickness are less sensitive to model assumptions and generally agree with the results obtained in previous studies

lidar detection of volcanic aerosols in the atmosphere following the mount st helen eruption we report lidar measurements of the aerosol content of the atmosphere for the first two months following the volcanic eruption of mt st helen the measurements were taken near l aquila italy with a lidar system utilizing a 50cm diameter telescope the measurements are compared to similar data for the previous quiescent period and show a large increase of the backscattering ratio in the stratosphere and upper troposphere layers have been observed at various altitudes a strong layer at 10 14km has been observed only initially layers between 14 25km have been observed continually these features show a considerable day to day variability

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

the many surface expressions of mantle dynamics plate tectonic theory suggests that present day topography can be explained by the repeated interactions between the tectonic plates moving along earth s surface however mounting evidence indicates that a significant proportion of earth s topography results from the viscous stresses created by flow within the underlying mantle rather than by the moving plates this dynamic topography is transient varying as mantle flow changes and is characterized by small amplitudes and long wavelengths it is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements however dynamic topography can influence surface processes and thus enter the geological record it has for example played a role in the establishment of amazon drainage patterns in turn surface processes such as the erosion of topographical anomalies could affect mantle flow this emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics

quest for dynamic topography observations from southeast asia transient dynamic topography is maintained by time varying density anomalies deep within earth9s mantle there is considerable interest in predicting and measuring dynamic topography because its spatial and temporal variation will help to track changes in the pattern of mantle convection despite the availability of increasingly elaborate predictive models there is little agreement about the amplitude wavelength and rate of change of dynamic topography here we analyze a hybrid data set of subsidence observations from continental and oceanic basins in southeast asia where published dynamic models predict 1 2 km of regional subsidence residual subsidence was isolated by removing the well defined effects of lithospheric extension and or cooling at long wavelengths 10 3 km the amplitude of residual subsidence i e maximum permissible dynamic subsidence has an upper bound of only 300 m the spatial distribution of permissible dynamic topography has no simple spatial or temporal relationship to the pattern of subduction over the past 65 m y anomalous subsidence commenced 5 10 m y ago accelerating to a present day rate of 100 m m y our results show that density anomalies within the lower mantle have little apparent influence at earth9s surface instead the distribution of residual subsidence reported here can be accounted for by minor variations in lithospheric thickness or by transient density variations beneath the lithospheric plate the absence of measurable dynamic topography is surprising and indicates that mantle mass anomalies are supported elsewhere presumably at internal boundaries within earth our hybrid data set can thus be used as an independent constraint on the viscosity structure of the mantle

the influence of dynamically supported topography on estimates of te abstract the effective elastic thickness t e of continental lithosphere can be estimated from the relationship between gravity and topography in the spectral domain two methods have been used one of which depends on the coherence between bouguer gravity anomalies and topography whereas the other uses the transfer function commonly known as the admittance between the free air gravity and topography the two methods give estimates of t e which differ by as much as an order of magnitude in those continental regions where variations in elevation are small this problem has led to much controversy an important concern is the extent to which estimates of t e are affected by dynamically maintained gravity and topography arising from mantle convection and postglacial recovery unlike elastically supported anomalies these processes can generate gravity and topographic long wavelength 500 km anomalies if such anomalies are modelled as being supported by elastic forces the resulting values of t e are overestimated often by a large amount

topographic asymmetry of the south atlantic from global models of mantle flow and lithospheric stretching the relief of the south atlantic is characterized by elevated passive continental margins along southern africa and eastern brazil and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank we investigate the origin of these topographic features in the present and over time since the jurassic with a model of global mantle flow and lithospheric deformation the model progressively assimilates plate kinematics plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates and predicts the evolution of mantle temperature continental crustal thickness long wavelength dynamic topography and isostatic topography mantle viscosity and the kinematics of the opening of the south atlantic are adjustable parameters in thirteen model cases model predictions are compared to observables both for the present day and in the past present day predictions are compared to topography mantle tomography and an estimate of residual topography predictions for the past are compared to tectonic subsidence from backstripped borehole data along the south american passive margin and to dynamic uplift as constrained by thermochronology in southern africa comparison between model predictions and observations suggests that the first order features of the topography of the south atlantic are due to long wavelength dynamic topography rather than to asthenospheric processes the uplift of southern africa is best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle

seismic evidence for significant crustal thickening beneath jabalpur earthquake 21 may 1997 source region in narmada son lineament central india 1 we investigate crustal properties beneath the deep crustal 35 km jabalpur earthquake of 21 may 1997 narmada son lineament nsl central india in search of a possible cause of stress accumulation in the region teleseismic receiver functions computed at nine digital seismographs along a 250 km long profile suggests moho downwarp to 52 km across the width of the lineament in contrast with an average 40 km depth elsewhere in addition the crust beneath the nsl has higher vp vs of 1 84 compared to 1 73 in the surrounding suggestive of a high density mafic mass at depth that compensates the crustal root also supported by small topographic variation 200 m across the lineament presence of such an anomalous mass in the deep crust may lead to gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre existing narmada son fault leading to the earthquake in deep crust

buildup of a dynamically supported orogenic plateau numerical modeling of the zagros central iran case study the iranian plateau is a vast inland region with a smooth average elevation of c 1 5 km formed at the rear of the zagros orogen as a result of the arabia eurasia collision i e over the last 30 35 myr this collision zone is of particular interest due to its disputed resemblance to the faster himalayan collision which gave birth to the tibetan plateau around 50 myr ago recent studies have suggested that a recent 10 5 ma slab break off event below central iran caused the formation of the iranian plateau here we test several hypotheses through large scale 3082 3 590 km numerical models of continental subduction mod els that incorporate a free upper surface erosion rheological stratification brittle elastic ductile rheologies and metamorphic phase changes density and physical properties and account for the specific crustal and thermal structure of the arabian and iranian continental lithospheres we test the impact of the transition from oceanic to continental subduction and the topographic consequences of the progressive slowdown of the convergence rate during continental subduction our results demonstrate the role of mantle flow beneath the overriding plate initiated as an indirect consequence of slab break off this flow creates a dynamic topography support during continental subduction and results in delamination of the overriding plate lithospheric mantle followed by isostatic readjustment hence of further uplift and maintenance of a plateau like topography without significant crustal thickening the slowdown of the convergence rate dur ing the development of the continental subduction collision phase largely contributes to this process by controlling the timing and depth of slab break off

continental lithosphere folding in central asia part ii constraints from gravity and topography abstract periodical sub parallel fold like structures observed in the western gobi have two characteristic dominant wavelengths of approximately 50 and 300 360 km we explain these observations in terms of independent quasi viscous folding of the crustal and upper mantle parts of the lithosphere due to horizontal transpressional stresses in central asia induced by the collision with the indian plate we derive a model for the mechanical response of the continental lithosphere to horizontal stresses for a rheologically layered plate with non newtonian power law rheology overlying a low viscosity asthenosphere the differentiation in the effective viscosity and thickness of the strong upper crust and upper mantle lithosphere along with the presence of a low viscosity lower crust between them leads to their partial decoupling during compressional deformation these features result in the appearance of different wavelengths of the folds in the western gobi region we derive simple semi analytical estimates for the dominant wavelengths and rates of growth of surface undulations constrained by data from experimental rock mechanics topography and gravity

mantle dynamics in the mediterranean the mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt lithospheric dynamics are affected by slab rollback and collision of two large slowly moving plates forcing fragments of continental and oceanic lithosphere to interact this paper reviews the rich and growing set of constraints from geological reconstructions geodetic data and crustal and upper mantle heterogeneity imaged by structural seismology we proceed to discuss a conceptual and quantitative framework for the causes of surface deformation exploring existing and newly developed tectonic and numerical geodynamic models we illustrate the role of mantle convection on surface geology a coherent picture emerges which can be outlined by two almost symmetric upper mantle convection cells the downwellings are found in the center of the mediterranean and are associated with the descent of the tyrrhenian and the hellenic slabs during plate convergence these slabs migrated backward with respect to the eurasian upper plate inducing a return flow of the asthenosphere from the backarc regions towards the subduction zones this flow can be found at large distance from the subduction zones and is at present expressed in two upwellings beneath anatolia and eastern iberia this convection system provides an explanation for the general pattern of seismic anisotropy in the mediterranean first order anatolia and adria microplate kinematics and may contribute to the high elevation of scarcely deformed areas such as anatolia and eastern iberia more generally the mediterranean is an illustration of how upper mantle small scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the tethyan collision

joint seismic geodynamic mineral physical modelling of african geodynamics a reconciliation of deep mantle convection with surface geophysical constraints recent progress in seismic tomography provides the first complete 3 d images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the african continent we present a tomography based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions where the latter are constrained by mineral physics the application of this thermochemical convection model to the problem of african mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep seated mantle flow under the african plate over a wider range of wavelengths than has been possible to date on the basis of these results we predict flow in the african asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late cenozoic volcanism including the kenya domes hoggar massif cameroon volcanicline cape verde and canary islands the flow predictions also reveal a deep seated large scale active hot upwelling below the western margin of africa under the cape verde islands that extends down to the core mantle boundary the scale and dynamical intensity of this west african superplume is comparable to the south african superplume that has long been assumed to dominate the large scale flow dynamics in the deep mantle under africa we evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of african shear wave splitting data

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

active deformation of asia from kinematics to dynamics using estimated strain rates across the actively deforming region of asia to infer variations in viscous stress it is shown that gradients of stress point uphill toward the center of tibet where gravitational potential energy per unit surface area reaches a maximum thus the dynamics of the deformation seem to obey the equation of creeping flow which expresses a balance between gradients in stress and the gravitational body force this balance in the region of the tibetan plateau yields an estimate of 10 22 pascal second for the average viscosity of the tibetan lithosphere which is only about 10 to 100 times greater than the viscosity of the convecting upper mantle

predicting and testing continental vertical motion histories since the paleozoic abstract dynamic topography at the earth s surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry sea level change continental flooding sedimentation and erosion these observations provide important constraints on and test of mantle dynamic models based on global mantle convection models coupled with the surface plate motion history we compute dynamic topography and its history for the last 400 ma associated with pangea assembly and breakup with particular focus on cratonic regions we propose that burial unroofing histories of cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models our models show that there are currently two broad dynamic topography highs in the pacific and africa for the present day earth that are associated with the broad warm structures i e superplumes in the deep mantle consistent with previous proposals of dynamical support for the pacific and african superswells our models reveal that pangea assembly and breakup by affecting subduction and mantle upwelling processes have significant effects on continental vertical motions our models predict that the slave craton in north america subsides before pangea assembly at 330 ma but uplifts significantly from 330 ma to 240 ma in response to pre pangea subduction and post assembly mantle warming the kaapvaal craton of africa is predicted to undergo uplift from 180 ma to 90 ma after pangea breakup but its dynamic topography remains stable for the last 90 ma the predicted histories of elevation change for the slave and kaapvaal cratons compare well with the burial unroofing histories inferred from thermochronology studies thus supporting our dynamic models including the development of the african superplume mantle structure the vertical motion histories for other cratons can provide further tests of and constraints on our mantle dynamic models

the isostatic stagnant lid approximation and global variations in the venusian lithospheric thickness we use the long wavelength venusian topography and geoid to constrain the global lithospheric thickness by assuming that the stagnant lid overlying the convective mantle is in isostatic equilibrium the isostatic stagnant lid isl approximation two dimensional and three dimensional numerical simulations of convection show that the isl approximation is reasonably accurate in the tested parameter range of temperature dependant viscosity convection with bottom heating and internal heating the errors in predicting the thickness of the stagnant lid i e the lithosphere are somewhat larger for internally heated convection we attribute this to a flatter lid and as a result to a larger relative contribution to topography and geoid anomalies from the deeper parts of the layer application of the isl approximation to venus suggests that the average lithospheric thickness might be as large as 600 km which is on the high end of previously proposed estimates

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

origins of topography in the western u s mapping crustal and upper mantle density variations using a uniform seismic velocity model to investigate the physical basis for support of topography in the western u s we construct a subcontinent scale 3 d density model using 1000 estimated crustal thicknesses and s velocity profiles to 150 km depth at each of 947 seismic stations crustal temperature and composition are considered but we assume that mantle velocity variations are thermal in origin from these densities we calculate crustal and mantle topographic contributions typical 2 uncertainty of topography is 500 m and elevations in 84 of the region are reproduced within error remaining deviations from observed elevations are attributed to melt variations in crustal quartz content and dynamic topography compositional variations in the mantle while plausible are not necessary to reproduce topography support for western u s topography is heterogeneous with each province having a unique combination of mechanisms topography due to mantle buoyancy is nearly constant within 250 m across the cordillera relief there 2 km results from variations in crustal chemistry and thickness cold mantle provides 1 5 km of ballast to the thick crust of the great plains and wyoming craton crustal temperature variations and dynamic pressures have smaller magnitude and or more localized impacts positive gravitational potential energy gpe anomalies 2 1012n m calculated from our model promote extension in the northern basin and range and near the sierra nevada negative gpe anomalies 3 1012n m along the western north american margin and yakima fold and thrust belt add compressive stresses stresses derived from lithospheric density variations may strongly modulate tectonic stresses in the western u s continental interior

geothermal anomalies in the crust and upper mantle along southern rocky mountain transitions heat flow temperature gradient and gravity data are analyzed along five profiles crossing from the colorado plateau or great plains to the high topographic areas of the southern rocky mountains these profiles cross large negative bouguer gravity anomalies associated with the high topography in the region and are within the area studied by two recent seismic investigations the heatflow and gravity profiles typically cross from maximum to minimum values in the region allowing an fft fast fourier transform algorithm to be applied to the data so that high frequencies can be filtered the resulting smoothed data can be fitted by the first few terms of a sine series to estimate mid depths of crustal and upper mantle step sources although anomalies could also be caused by different source geometries at much shallower or much deeper depths the analysis provides a depth comparison for thermal and gravity anomaly sources modeled with similar geometries and a comparison of thermal source depths with seismic velocity anomalies there is considerable similarity along the profiles between the gravity and heat flow data and anomaly sources appear to be both in the crust and upper mantle there often appears to be spatial coincidence between thermal and gravity anomaly sources and seismic velocity anomalies proposed in the upper mantle implying thermal influence to these gravity and seismic anomalies thermal anomalies proposed to have sources in the crust in some areas are consistent with gravity and shear wave anomalies and support studies in those areas suggesting high crustal radiogenic concentrations in relatively less dense granitic crustal rocks

the role of boundary mixing in the deep ocean recent laboratory studies demonstrate that mixing at the boundaries of stratified and rotating fluids can be described by a relatively simple parameterization these results are applied to the deep ocean and by using available field observations it is argued that mixing at the boundaries can lead to an apparent diapycnal diffusivity in the interior of the deep ocean whose magnitude has an upper bound of 3 10 5 m2 s 1

the climatology of the north atlantic abstract data from approximately 144 000 hydrographic stations in the north atlantic have been retrieved from the national oceanic data centre and analysed to produce maps of mean pressure temperature salinity and oxygen on selected potential density surfaces for the domain bounded by 0 85 w and 0 65 n the data span the period from 1904 to 1990 with the majority of the data from the last four decades the data set for this region is 60 larger than that used in the production of levitus climatological world atlas this increase in stations coupled with smoothing scales specific to the north atlantic rather than the global ocean considerably improves the resolution of the basin s features the mean property fields and their associated standard deviations are resolved on a one degree grid with little smoothing contrasting with the levitus atlas where properties although presented on a one degree grid have been smoothed on the order of 1000km another important feature of this database is the process by which irregularly spaced data are averaged onto a regular grid in a significant departure from the levitus analysis which averaged on depth surfaces these data were averaged on potential density surfaces thus eliminating an artificial mixing of water mass properties the database is used to describe the baroclinicity of well known features such as the gulf stream the north atlantic current and the deep western boundary current and to elucidate the recirculations associated with these currents it additionally resolves several new features in the intermediate and deep north atlantic these features include the signature of a large scale deep recirculation that extends southwestward from the eastward extension of the north atlantic current to the separation point of the gulf stream near cape hatteras this recirculation which spans approximately 2000m of the water column encompasses more local recirculations and potentially mixes subpolar and subtropical waters furthermore in the upper thermocline this database reveals a coherent azores current that stretches from the gulf stream system south of the tail of the grand banks to madeira this flow is marked by divergences to the south and convergences from the north such that its downstream transport is not much changed

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

abyssal currents and advection of resuspended sediment along the northeastern bermuda rise abstract an integrated study using current meters time series nephelometers and water samplers for measurements of particulate matter concentration revealed an intense nepheloid layer centered on the slopes of the eastward scarp the northeastern boundary of the bermuda rise these slopes also contained extensive furrows truncated seismic reflectors and wisconsin age and older surface sediments the abyssal circulation in this region appears to be accelerated when it encounters the steep topography of the eastward scarp eroding sediments creating the erosional furrows and moving eroded sediment upslope instruments on top of the bermuda rise recorded episodic events benthic storms where particle concentrations at 25 m above the bottom increased from a background level of 60 g 1 1 to maximum of 251 g 1 1 and current velocities were nearly 20 cm s 1 net transport of both water and entrained sediment was to the west northwest during the largest benthic storm water temperature decreased by 0 1 c consistent with a 300 600 m upslope transport of water and resuspended sediments from the upper slopes of the eastward scarp when currents carry resuspended sediment over the rim of the eastward scarp to the plateau region the current slows depositing its sediment load thus the plateau is growing upwards slowly at the expense of the surrounding scarps

upper mantle shear structure beneath the colorado rocky mountains a tomographic inversion of teleseismic shear wave delays has been performed using data collected in the 1992 program for array seismic studies of the continental lithosphere rocky mountain front experiment the shear wave residuals used as data were corrected for known crustal sediment and topographic differences and thus were sensitive to mantle shear velocity variations beneath the colorado array large differences in shear wave travel time are observed across colorado and into western kansas with a maximum difference of near 5 sec between the central rockies in colorado and a station in western kansas the shear wave residuals were inverted for shear wave velocity variations in the upper 350 km of the mantle beneath colorado and kansas large differences in mantle shear velocity are found between the rockies and the kansas great plains the average shear velocity in the upper 200 km is about 9 greater beneath kansas than beneath the rockies at greater depths lateral variations are far smaller and not as well correlated with surface tectonics in a qualitative sense there appear to be three mantle structures present beneath the study area beneath the rockies the upper mantle is extremely slow both to the west and east of the rockies we find intermediate mantle shear velocity there is then an abrupt increase in mantle shear velocity near the colorado kansas border we interpret the abrupt increase in mantle shear velocity near the colorado kansas border to mark the boundary of cratonic lithosphere the slow velocity zone beneath the rockies appears to be upwelled asthenosphere as is seen beneath the rio grande rift to the south to match the seismic contrast between the mantle beneath kansas and the colorado rockies requires a temperature contrast of at least 350 c as well as partial melt beneath the rockies the hot mantle beneath the rockies implies that the mantle density is low and the high topography of the rockies is at least partially due to buoyant mantle

lithospheric velocity structure of the anatolian plateau caucasus caspian region 1 the anatolian plateau caucasus caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities despite the complexity of the region little is known about the detailed lithospheric structure using data from 31 new permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region a 3 d velocity model is developed using joint inversion of teleseismic receiver functions and surface waves both group and phase dispersion curves love and rayleigh were derived from regional and teleseismic events additional rayleigh wave group dispersion curves were determined using ambient noise correlation receiver functions were calculated using p arrivals from 789 teleseismic 30 90 earthquakes the stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station the depths of major discontinuities sediment basement crust mantle and lithosphere asthenosphere were inferred from the velocity depth profiles at the location of each station distinct spatial variations in crustal and upper mantle shear velocities were observed the kura basin showed slow 2 7 2 9 km s upper crustal 0 11 km velocities but elevated 3 8 3 9 km s velocities in the lower crust the anatolian plateau varied from 3 1 3 2 in the upper crust to 3 5 3 7 in the lower crust while velocities in the arabian plate south of the bitlis suture were slightly faster upper crust between 3 3 and 3 4 km s and lower crust between 3 8 and 3 9 km s the depth of the moho which was estimated from the shear velocity profiles was 35 km in the arabian plate and increased northward to 54 km at the southern edge of the greater caucasus moho depths in the kura and at the edge of the caspian showed more spatial variability but ranged between 35 and 45 km upper mantle velocities were slow under the anatolian plateau but increased to the south under the arabian plate and to the east 4 3 4 4 km s under the kura basin and greater caucasus the areas of slow mantle coincided with the locations of holocene volcanoes differences between rayleigh and love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the anatolian plateau and the kura basin

mantle convection and the recent evolution of the colorado plateau and the rio grande rift valley the colorado plateau contains late cretaceous marine strata that are at a mean elevation of 2 km the timing and amount of uplift since the cretaceous has generated considerable debate with the exception of a few studies topography supported by vertical stresses generated by viscous fl ow in the mantle has not been explicitly considered to contribute to the elevation of this region herein we compute the viscous fl ow beneath north america that is driven by density anomalies inferred from joint seismic geodynamic modeling we fithat the colorado plateau overlies a strong mantle upwelling that is coupled to the sinking farallon slab currently beneath the eastern united states consequently the colorado plateau is currently a focused dynamic topography high within the western u s cordillera moreover this strong upwelling impacts the base of the lithosphere at an oblique angle east of the plateau directly below the rio grande rift we attribute this fl ow as being responsible for some of the recent magmatic activity along the jemez lineament as well as contributing to the recent rifting process in the rio grande rift valley

dynamics of the bottom boundary layer on the northern california shelf abstract time series measurements of velocity temperature and conductivity on the northern california shelf during two winter seasons permit an observational test in vertically integrated form of a simple set of subinertial momentum and heat balances for the bottom boundary layer which have resulted from recent theoretical work these are 1 an along isobath momentum equation that reduces to a classic ekman balance 2 a cross isobath momentum equation in which the ekman balance is modified by a buoyancy force caused by distortion of the isopycnal surfaces within the boundary layer and 3 a heat balance in which variability of temperature is produced by cross isobath advection the measurements confirm the importance of buoyancy in the cross isobath momentum equation and as has recently been predicted theoretically they indicate that buoyancy is a dominant effect when the boundary layer is thick which typically occurs during downwelling favorable flows an ekman balance describes subinertial flu

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

formation of sediment waves by turbidity currents and geostrophic flows a discussion abstract a condition for the existence of sediment waves under turbidity currents as antidunes with the requirement of a slope gradient 3 0 10 3 is deduced data show no such waves on slopes 3 but some contourite mudwaves occur on slopes as low as 4 4 10 4 however the latter also occur on slopes 3 10 3 so no clear distinction is possible where turbidity current channels cross sediment drifts or geostrophic flows traverse turbidite fans the origin of most mudwaves will need to be determined by reference to internal features and context a key problem is deposition of mud as antidunes from turbidity currents where even the waning flow is probably well above the critical erosion velocity for a clear flow deposition must occur from high concentration flows well above clear water critical depositional stresses once a wavy bed is set up subsequent deposition may occur via the lee wave mechanism proposed for contourite waves under a gradient froude number 1 a steep angle

collection of microparticles at high balloon altitudes in the stratosphere we report the collection of stratospheric particles at 34 36 km using balloon borne collectors the concentration of particles on the collection surfaces and element concentrations were measured on the majority of the particles using scanning electron microscope sem and proton induced x ray emission pixe analysis respectively particle morphologies elemental composition and electron diffraction data were obtained on a small number of the collected particles using transmission electron microscope tem techniques the concentration of particles between 0 045 lower imaging limit and 1 m in radius is several orders of magnitude above the blank levels on the collection surfaces while the concentration of particles above 1 m is near blank levels more than 106 submicron particles were collected the concentration of submicron particles is between 10 and 50 times the concentration expected at the sampling altitude based on models using the measured flux of extraterrestrial particles to the atmosphere the higher concentration of submicron particles may be due to contributions from volcanic particles inaccuracies in the influx of particles of this size to the earth or breakup of larger particles analysis of the elemental composition using pixe showed cl s ti fe br ni zr zn sr and cu in decreasing order of concentration elements below s could not be detected in the analysis cl s and br are believed to be present due to reaction of the collection surfaces and particles with atmospheric gases while the other elements are present in the particles the elemental composition of the particles does not allow an unequivocal origin to be assigned because particles from several of the possible sources listed above contain the measured elements tem analysis of 23 of the submicron particles showed 16 to be non graphitic the particles ranged from al rich silicates to almost pure fe to one containing almost exclusively ba and s none were definitely chondritic in composition this collection of particles complements existing collections obtained by aircraft they were colleted at much higher altitude and are predominantly submicron in size collection and analysis of particles at high altitudes using balloons presents unique challenges and opportunities in understanding the particle population in the stratosphere

mantle dynamics isostasy and the support of high terrain dynamic topography is commonly understood to be deflection of the earth s surface that results from convection of the mantle because different authors use the words dynamic topography differently topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m for most regions however surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography the small free air gravity anomalies and still smaller isostatic anomalies 2000 m of dynamic topography moreover simple examples show that if flow induced stresses maintain a given density distribution then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium in light of confusion over the term dynamic topography we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow induced stresses and to distinguish it from topography that is compensated isostatically

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

images of crustal variations in the intermountain west 1 we develop a map of crustal thickness variations across the great basin colorado plateau rocky mountain and great plains provinces of the western united states using common conversion point stacking of teleseismic receiver functions below the rocky mountains and high plains in colorado we find the thickest crust in the region at 45 50 km thick beneath the basin and range thinner between 30 and 40 km crust is found thin 30 km thick crust is present in the northern portion of nevada and utah despite elevations similar to those farther south crustal thickness across the colorado plateau can be characterized as a broad transitional region between the thin crust of basin and range to the thicker crust of the rocky mountains the impedance contrast across the mohorovicic discontinuity decreases below the colorado plateau as converted arrivals recorded in this region appear weak compared to surrounding areas variations in vp vs across the region indicate higher values along the western boundary of the basin and range in the rocky mountains and in the western great plains we are not able to characterize vp vs in the colorado plateau we find that crustal thickness does not closely correlate with surface topography within each region or across the region as a whole differences in crustal thickness in each tectonic province indicate the need for a mantle component to support the high elevations across the western united states

a study of the bottom boundary layer over the eastward scarp of the bermuda rise velocity and temperature measurements from the bottom boundary layer bbl over the eastward scarp of the bermuda rise in water depth of 4620 m show little variability over an 8 month period the free stream flow 62 m above the bottom was south southeasterly following the isobaths in the region with an average speed of 22 cm s the current vector in the bbl rotated an average of 5 in a counterclockwise sense between 62 and 0 8 m above the bottom the thickness of the bbl was 40 m and the average magnitude of the bottom stress was 0 7 dynes cm2 mean speed profiles height of the bbl and the magnitude of the bottom stress predicted by a model compare favorably with the observations but the model predicts a rotation of the current vector between 62 and 0 8 m more than twice that measured the time dependent nature of the flow field is also reproduced by the model the bermuda rise data and speed profile measurements at the base of the scotian rise show that the m2 clockwise polarized tide is damped more than the mean current as the bottom is approached in the bbl this phenomenon is reproduced by the model and can be explained by differing effective ekman layer thicknesses associated with tidal and steady components of the flow

the relationship between topography and gravity on earth and venus abstract the measurement of the admittance between the topography and the gravity of a planet as a function of wavelength provides important constraints on its interior processes a detailed discussion of the influence of noise on such measurements shows that at long wavelengths the admittance is best determined in the frequency domain using gravity as input and topography as output this procedure is used to estimate the admittance in the pacific and indian oceans the observed values are reduced by volcanism above plumes which produces a contribution to the long wavelength compensated topography when this effect is taken into account there is little evidence that the admittance is reduced by the presence of a low viscosity zone in the upper mantle a similar study of venus shows an excellent correlation between gravity g and topography h for wavelengths greater than 700 km with an admittance of about 50 mgal km 1 this value agrees well with that expected from numerical axisymmetric models of plumes and can be used to construct maps of residual topography by calculating h g 50 where h is in kilometers and g in milligals the power spectrum of the gravity field and its skewness with positive anomalies covering a smaller area but being larger than the negative ones are also in agreement with those of the numerical models of the major topographic features only aphrodite and ishtar terrae behave differently with a value of admittance of about one third of other large structures this difference requires their topography to be partly supported by variations in crustal thickness by analogy with similar structures on earth both are likely to be supported by volcanically thickened crust on top of rising plumes though the observed relationships do not rule out other models the behavior of the admittance at long wavelengths is important because it can constrain the thickness of the convecting layer at wavelengths of more than 3000 km the gravity data provides some evidence that the layer depth is about 1000 km over the center of beta and phoebe regiones there are short wavelength anomalies of residual topography produced by crust that is thinner than that elsewhere flanked by positive anomalies of thicker crust the regions of thin crust occur beneath rift zones this behavior can be explained if crust flows down the flanks of elevated regions above plumes and can generate coronae when the plume support disappears

lithospheric structure of the aegean obtained from p and s receiver functions combined p and s receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the aegean region are used to map the geometry of the subducted african and the overriding aegean plates we image the moho of the subducting african plate at depths ranging from 40 km beneath southern crete and the western peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern greece however the dip of the moho of the subducting african plate is shallower beneath the peloponnesus than beneath crete and rhodes and flattens out beneath the northern aegean observed p to s conversions at stations located in the forearc indicate a reversed velocity contrast at the moho boundary of the aegean plate whereas this boundary is observed as a normal velocity contrast by the s to p conversions our modeling suggests that the presence of a large amount of serpentinite more than 30 in the forearc mantle wedge which generally occurs in the subduction zones may be the reason for the reverse sign of the p to s conversion coefficient moho depths for the aegean plate show that the southern part of the aegean crustal thickness of 20 22 km has been strongly influenced by extension while the northern aegean sea which at present undergoes the highest crustal deformation shows a relatively thicker crust 25 28 km this may imply a recent initiation of the present kinematics in the aegean western greece crustal thickness of 32 40 km is unaffected by the recent extension but underwent crustal thickening during the hellenides mountains building event the depths of the aegean moho beneath the margin of the peloponnesus and crete 25 28 and 25 33 km respectively show that these areas are also likely to be affected by the aegean extension even though the cyclades crustal thickness of 26 30 km were not significantly involved in this episode the aegean lithosphere asthenosphere boundary lab mapped with s receiver functions is about 150 km deep beneath mainland greece whereas the lab of the subducted african plate dips from 100 km beneath crete and the southern aegean sea to about 225 km under the volcanic arc this implies a thickness of 60 65 km for the subducted african lithosphere suggesting that the aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the cyclades

regional compensation of subducted lithosphere effects on geoid gravity and topography from a preliminary model abstract it is proposed that the anomalous mass of subducted lithospheric slabs is partially compensated by two mechanisms a by stresses transmitted up the slab to the surface lithosphere causing the trench and b by stresses transmitted through the mantle wedge overlying the slab which cause a broad shallow depression results calculated from a preliminary two dimensional model show that it can give a first order account of observed topography gravity and geoid anomalies in particular the model can reconcile the large mass anomaly predicted for the slab by thermal conduction theory with the smaller than expected gravity and geoid anomalies preliminary results indicate substantial compensation by mechanism a implying rather large stresses at the top of the slab these could be important for theories of earthquake mechanisms and plate driving mechanisms roughly 50 of the possible compensation by mechanism b is indicated the resulting depression the order of 500 m deep and over 500 km wide could explain anomalously deep marginal basins and some marine transgressions of continents

dynamic subsidence and uplift of the colorado plateau we use inverse models of mantle convection to explore the vertical evolution of the colorado plateau by satisfying multiple constraints seismic tomography stratigraphy in the western united states and great plains and other structural and volcanic data adjacent to the plateau the model provides predictions on the continuous history of colorado plateau vertical motion since 100 ma with the arrival of the flat lying farallon slab dynamic subsidence swept from west to east over the plateau and reached a maximum ca 86 ma two stages of uplift followed the removal of the farallon slab below the plateau one in the latest cretaceous and the other in the eocene with a cumulative uplift of 1 2 km both the descent of the slab and buoyant upwellings raised the plateau to its current elevation during the oligocene a locally thick plateau lithosphere enhances the coupling to the upper mantle so that the plateau has a higher topography with sharp edges the models predict that the plateau tilted downward to the northeast before the oligocene caused by northeast trending subduction of the farallon slab and that this northeast tilting diminished and reversed to the southwest during the miocene in response to buoyant upwellings

dynamic surface topography a new interpretation based upon mantle flow models derived from seismic tomography the very long wavelength i e in the degree range l 1 8 dynamic topography at the earth s surface estimated by isostatic reduction of the observed topography is shown to be in good agreement with the topography predicted by a model of viscous flow in the mantle this flow model was previously derived by independent consideration of the observed nonhydrostatic geoid and is based upon a recent model of large scale seismic shear velocity heterogeneity in the mantle our estimate and prediction of the long wavelength dynamic surface topography reveals significant depressions 2 3 km of the continental shields relative to their hydrostatic reference positions

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

lidar observations of dust layers transience in the stratosphere following the el chichon volcanic eruption intense stratospheric dust layers have been observed with a lidar at l aquila 42 n 13 e during the months of july and august 1982 these features were located at 22 26km with a maximum scattering ratio of 13 in one occasion a layer was also observed at 32km maximum integrated scattering is of the order of 0 001 sr 1 the almost daily frequency of the observations has revealed an unusually fast decrease of the altitude of the layers with an apparent velocity of 0 2 0 3km day this pattern is repeated in both the high altitude layers observed it is speculated that this effect could be attributed to the zonal distribution of the dust which in turn could depends on dust formation process

an analog experiment for the aegean to describe the contribution of gravitational potential energy the southern aegean seafloor exhibits clear evidence of internal deformation stretching as shown by tectonics seismology and space geodesy we use an analog three layer laboratory experiment of sand silicone putty and honey to investigate the deformation of the southern aegean lithosphere the model is installed in a box and confined by a vertical wall we open a gate in the wall and observe the deformation of the two upper layers due to buoyancy forces the general pattern of the deformation of the southern aegean is found in the analog model we observe the formation of an arc spreading outward with time the extension is radial in the inner part but parallel to the arc in the external part and of comparable importance at both ends of the gate we observe strike slip motion dextral in the western part sinistral in the eastern part rotation clockwise in the western part counterclockwise in the eastern part of up to 40 is seen on both sides of the gate but is also present with a smaller amplitude far in the internal region partially due to distributed shear the spreading is associated with the thinning of the two upper layers and affects a region of dimensions comparable to the length of the free boundary this spreading does not propagate inward with time some pieces of material located near the active boundary remain undeformed during the experiment

steady propagation of delamination events delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation accelerating as the sinking slab of detached lithosphere grows longer this propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings the form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material without coupling the tip of the intrusion cools and stiffens to form a wedge dividing the crust cold mode with coupling the intrusion is forced to convect and remains ductile hot mode the cold mode can propagate at all velocities the hot mode has a lower limiting velocity of 1 2 cm year but offers less resistance at higher speeds resistance to delamination includes a constant term from the buoyant crustal downwarp plus a velocity proportional term representing viscous deformation however the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities matching this resistance to loading lines of 100 to 800 km slabs sinking in a mantle of 1022 p velocities of 0 3 8 0 cm year are obtained changes in viscosity affect this rate but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab the surface expression of delamination is a leading outer rise followed by a submarine trough with a large negative free air anomaly which finally evolves into a 1 km plateau if crustal viscosity and velocity are both low however there is a montonic crustal uplift with no trough thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm year driven by slabs up to 400 km in length

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

effects of a low viscosity lower crust on topography and gravity at convergent mountain belts during gravitational instability of mantle lithosphere we examine the gravitational stability of a stratified structure consisting of a low density crust through which viscosity decreases exponentially over a denser mantle lithosphere of constant viscosity which in turn overlies an inviscid slightly less dense layer like the asthenosphere the most important aspect of the viscosity structure is the contrast in viscosity at the moho surface uplift above a mantle downwelling is greatest when the viscosity contrast at the moho is negligible the surface is depressed when the viscosity of the lowermost crust is much greater than mantle viscosity for a wide range of viscosity structures calculated gravity anomalies and in particular admittances ratios of the fourier transform of gravity to that of topography produced by rayleigh taylor instability differ markedly from those observed across convergent mountain belts in particular whereas values of calculated admittance are negative or very small observed admittances are consistently positive thus if downwelling of mantle lithosphere directly beneath mountain ranges plays an important role in the construction of mountain belts it cannot be the main process that induces crustal thickening

anomalous uplift of the apennines and subsidence of the adriatic the result of active mantle flow 1 we consider the potential for interpreted mantle flow to generate anomalous topography across the north central mediterranean a seismic tomography section was translated into the temperature domain and used as input into a series of 2d thermo mechanical numerical models of crust mantle deformation the generated dynamic topography signal was then compared with a derived residual topography that showed a distinct residual low across the adriatic sea and easternmost apennines and a residual high to the west of the apennine range and into the tyrrhenian sea a good spatial correlation was found between the modeled dynamic effects of inferred underlying mantle structure and the observed residual topography the results of this study suggest that underlying mantle structure may play a role in supporting the dynamic anomalous surface topography along the investigated profile and within the north central mediterranean

u th he thermochronometry constraints on unroofing of the eastern kaapvaal craton and significance for uplift of the southern african plateau the timing and causes of the 1 0 km elevation gain of the southern african plateau since paleozoic time are widely debated we report the first apatite and titanite u th he thermochronometry data for southern africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau the study area encompasses 1500 m of relief within archean basement of the barberton greenstone belt region of the eastern kaapvaal craton titanite dates are neoproterozoic apatite dates are cretaceous with most results clustering at ca 100 ma thermal history simulations confirm mesozoic heating followed by accelerated cooling in mid to late cretaceous time the lower temperature sensitivity of the apatite u th he method relative to previous thermochronometry in southern africa allows tighter constraints on the cenozoic thermal history than past work the data limit cenozoic temperatures east of the escarpment to 35 c and appear best explained by temperatures within a few degrees of the modern surface temperature these results restrict cenozoic unroofing to less than 850 m and permit negligible erosion since the cretaceous if substantial uplift of the southern african plateau occurred in the cenozoic as advocated by some workers then it was not responsible for the majority of post paleozoic unroofing across the eastern escarpment significant mesozoic unroofing is coincident with large igneous province activity kimberlite magmatism and continental rifting within and along the margins of southern africa compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events

the benthic boundary layer and sediment transport the major problems facing scientists studying marine boundary layer fluid mechanics in 1979 concerned the specific mechanismic operating in the regions of the top and the bottom of the boundary layer at the top the mechanisms of fluid escapement from the boundary layer and mixing into the ocean interior could only be postulated approximations of diffusivity across the top of the layer were being debated the values of which were more articles of faith than physically validated estimates cf armi 1979 garrett 1979 at the bottom the intermittent generation of stress cf gordon 1975 heathershaw and simpson 1979 was held responsible for the wide variability observed in the drag coefficient and the thickness of the viscous sub layer was claimed to be greater than that traditionally found from laboratory studies caldwell and chriss 1979

genesis of collision volcanism in eastern anatolia turkey late cenozoic volcanism in eastern anatolia extends in a broad sw ne trending belt across the arabia eurasia collision zone from the arabian foreland basin in the southwest and to the kars plateau and lesser caucasus in the northeast foreland volcanism is dominated by basaltic shield and fissure eruptions of transitional tholeiitic alkaline composition volcanism on the thickened crust north of the bitlis thrust zone varies from the mildly alkaline volcano nemrut and older mus volcanics in the south through the transitional calc alkaline alkaline volcanoes bingol and suphan and the alkaline volcano tendurek to the calc alkaline volcano ararat and older kars plateau volcanics in the north isotope sr nd and trace element systematics indicate that the lavas from the foreland were derived from the mantle lithosphere of the arabian continent which had been enriched by small volumes of asthenospheric melts over a period of time and that lavas from the alkaline volcanic area around mus and the volcanoes nemrut and tendurek north of the bitlis thrust zone were derived from a lithospheric source of similar composition either from the same underthrust arabian continent or from the bitlis massif microcontinent by contrast the transitional lavas from bingol and suphan and the calc alkaline lavas from ararat and kars were derived from lithosphere carrying a distinct subduction signature inherited from pre collision subduction events positive correlations between 87sr 86sr and sio2 and rb nb and sio2 in the alkaline and transitional lavas suggest that combined assimilation and fractional crystallization was an important process within at least part of the thickened crust of the collision zone trace element covariation diagrams such as y rb indicate the importance of hornblende crystallization at depth and orthopyroxene at shallow levels within the calc alkaline provinces in contrast to the consistently anhydrous crystallization sequences of the alkaline lavas trace element diagrams based on the covariation of compatible and incompatible elements point to moderate low degrees of partial melting with residual clinopyroxene throughout and residual garnet in the foreland province consideration of mineral stabilities mantle solidi and geothermal gradients before and after collision suggest that lithospheric thickening should both increase the thickness of metasomatized lithosphere and depress the metasomatized zone to greater depths probably beneath the amphibole and dolomite breakdown curves perturbation of the thickened lithosphere by delamination of the thermal boundary layer perhaps coupled with local stretching associated with pull apart basins on strike slip fault systems is then sufficient to generate melt the composition of that melt being largely dependent on the enrichment history of the lithosphere in question in eastern anatolia volcanism appears to have started between about 8 and 6 ma ago some 5 ma after the start of rapid uplift of the east anatolian plateau

the nature of the crust in southern india implications for precambrian crustal evolution 1 we present crustal thickness and poisson s ratio determinations from receiver function analyzes at 32 sites on the archaean and proterozoic terrains of south india the crustal thickness in the late archaean 2 5 ga eastern dharwar craton varies from 34 39 km similar crustal thickness is observed beneath the deccan volcanic province and the cuddapah basin the most unexpected result is the anomalous present day crustal thickness of 42 51 km beneath the mid archaean 3 4 3 0 ga segment of the western dharwar craton since the amphibolite grade metamorphic mineral assemblages 5 7 kbar paleopressures in this part of western dharwar craton equilibrated at the depths of 15 20 km our observations suggest the existence of an exceptionally thick 57 70 km crust 3 0 ga ago beneath the exhumed granulite terrain in southernmost india the crustal thickness varies between 42 60 km the poisson s ratio ranges between 0 24 0 28 beneath the precambrian terrains indicating the presence of intermediate rock type in the lower crust these observations of thickened crust suggest significant crustal shortening in south india during the archaean

gravitational potential energy and active deformation in the apennines we use velocity measurements from a network of continuous gps sites spanning the apennines of peninsular italy to test the hypothesis that the active deformation of the region is explained by variations in gravitational potential energy of the lithosphere the simple geometry of the mountain chain allows us to treat the deformation as two dimensional neglecting gradients of velocity along the strike of the chain under this assumption the integral of gravitational potential energy per unit area of the lithosphere gpe in the direction perpendicular to the chain is related by a simple expression to the velocity in the same direction we show that the observed velocities match this expression with an rms misfit of 0 5mm yr this agreement suggests that deformation of the apennines reflects a balance within the mountain chain itself between lateral variations in gpe and the stresses required to deform the lithosphere forces arising from processes external to the belt are not required to explain the observations

a stratospheric aerosol increase during 1981 observed by lidar over mid europe after the middle of may 1981 lidar observations at garmisch partenkirchen recorded a marked enhancement of the aerosol backscattering from the lower stratosphere until the end of july several distinct aerosol layers could be detected which thereafter merged into a broad homogeneous layer the eruption of volcano alaid kurile islands on 28 april is suggested as the source of this recent stratospheric aerosol increase this aerosol disturbance is of about the same magnitude as that caused by mount st helens in may 1980 significant climatic effects are not predicted

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

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

estimating the compensation depth of the hawaiian swell with linear filters broad areas of anomalously shallow seafloor surround regions of active or recently active hot spot volcanism the cause of these midplate swells has been modeled both as elevated temperatures in a convecting layer and as thermal expansion within the conducting portion of the lithosphere according to the former explanation we would expect the low density material compensating the swell to lie beneath the conducting lid whereas the latter model allows compensation at shallower depths we present a technique based on the application of linear filters for determining the compensation depth of midplate swells given observations of topography and geoid or gravity anomalies our method does not require an a priori estimate of swell height or width nor does it assume complete spatial or spectral separation between the swell topography and the signal from the hot spot volcanoes which are compensated by flexure of the oceanic moho rather we exploit the difference in the predicted sign and amplitude of the moho deflection beneath elevated topography produced by loading above versus loading below the elastic plate in order to separate the two effects an application of these linear filters to residual topography and geoid data from the region surrounding the hawaiian hot spot yields a compensation depth of 70 10 km within the lower lithosphere

observation of a thin layer of material in the upper stratosphere in this letter we report the results of laser radar measurements at aberystwyth 52 25 n 4 04 w which revealed the presence of a discrete layer of atmospheric material of about 0 5 km thickness near 40 km for at least 3 h during the night of 4 5 july 1983 wind data indicate that it was moving from the east and had an east west extent of at least 170 km we estimate that the layer had a mass content in excess of 100 kg for each degree of latitude width this layer does not appear to be due to a natural terrestrial source or extraterrestrial material and it seems likely that it was a result of pollution at upper stratospheric heights

mantle flow uplift of western anatolia and the aegean interpretations from geophysical analyses and geodynamic modeling 1 the western anatolian and aegean region demonstrates a complex geologic history of horizontal and vertical tectonics active normal faulting and exhumation zones indicate that western anatolia has experienced significant extension since the oligocene early miocene 30 ma our geophysical analyses demonstrate that the region is also uplifted relative to an elevation that would be expected given an isostatic response to the lithosphere structure namely topography residuals indicate a residual uplift of about 1500 m over 200 km sections of western anatolia and the aegean admittance functions between free air gravity and topography indicate that the regional topography is isostatically uncompensated and as it approaches 50 mgal km at the longest wavelengths the uncompensated topography is likely owing to an underlying mantle flow component using forward geodynamic modelling we consider an idealized section of western anatolian lithosphere based on tomographic inversions and examine the magnitude and pattern of surface topography to reconcile with the geophysical observables the models consistently show a plateau type uplift and horizontal extension through western anatolia with an amplitude and wavelength consistent with the residual topography calculations together the geophysical analyses and modelling provide independent quantitative evidence that the thin anatolian aegean lithosphere is being buoyed upwards by underlying mantle flow the mantle flow may be associated with active lithosphere delamination beneath the region a process that would also explain the ongoing crustal extension

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

eucrust 07 a new reference model for the european crust 1 we present a new digital model eucrust 07 for the crust of western and central europe and surroundings 35n 71n 25w 35e available results of seismic reflection refraction and receiver functions studies are assembled in an integrated model at a uniform grid 15 0 15 0 the model consists of three layers sediments and two layers of the crystalline crust besides depth to the boundaries we provide average p wave velocities in the upper and lower parts of the crystalline crust the new model demonstrates large differences in the moho depth compared to previous compilations over 10 km in some specific areas e g the baltic shield furthermore the velocity structure of the crust is much more heterogeneous than in previous maps eucrust 07 offers a starting point for numerical modeling of deeper structures by allowing correction for crustal effects beforehand and to resolve trade off with mantle heterogeneities citation tesauro m m k kaban and s a p l cloetingh 2008 eucrust 07 a new reference model for the european crust geophys res lett 35 l05313 doi 10 1029 2007gl032244

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

rapid erosion of the southern african plateau as it climbs over a mantle superswell we present new sedimentary flux data confirming that a large pulse of erosion affected the southern african plateau in the late cretaceous and is likely to be related to a major uplift episode of the plateau this short phase of erosion i e less than 30 myr in duration has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift given its size the rise of the african superplume is likely to have lasted much longer here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent wide drainage reorganization leading to substantial denudation in a relatively short amount of time the amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the karoo sedimentary and volcanic rocks and the underlying basement we also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse suggesting that climate may have also played a role in triggering the denudation the diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are however better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling