the coherence method using a thin anisotropic elastic plate model 1 the coherence method based on the statistical correlation between gravity and topography has been widely used to estimate the effective elastic thickness of the continental lithosphere and its variations by far the majority of studies have assumed an isotropic lithosphere but several papers have inferred anisotropy from differences in the observed coherence or admittance functions in different directions however none have used an anisotropic model to fit the coherence we show how the simplest model of elastic anisotropy an orthotropic thin plate can be incorporated into both forward and inverse modeling algorithms using an appropriate approximation in the partial differential equation for the flexure of such a plate

elastic thickness of the iranian lithosphere from gravity and seismic data abstract we estimate the effective elastic thickness of the iranian lithosphere and adjoining tectonic plates by using the approach that combines the vening meinesz moritz s vmm regional isostatic principle with the isostatic flexural model formulated based on solving a flexural differential equation for a thin elastic shell to model the response on a load more realistically we also consider the lithospheric density structure the resulting expression describes a functional relation that links gravity field and mechanical properties of the lithosphere the young modulus and the poisson ratio are computed from seismic velocity data in prior of estimating the lithospheric elastic thickness the presented results reveal that the estimated elastic thickness closely resembles a regional tectonic configuration associated with the extensional tectonism along the red sea gulf rift system the continental collision of the arabian and eurasian plates and the subduction along the makran subduction zone seismically and volcanically active convergent tectonic margins of the zagros and kopeh dagh fold and thrust belts further extending along the makran accretionary complex are characterised by a low lithospheric strength with the elastic thickness typically less than 30 km these small values of the elastic thickness are in a striking contrast to much larger values within most of the central iranian blocks according to our estimate local maxima there reach 70 km in the tabas micro block the elastic thickness of the turan and arabian platforms reaches maxima of 100 km these results generally support the hypothesis that tectonically active zones and orogens have a relatively low strength resulting in a significant response of the lithosphere on various tectonic loads compared to a significant strength of old cratonic formations interestingly however we observe a striking contrast between a low strength of the arabian shield compared to a high strength of the arabian platform a possible explanation of this finding could be given by a different thermal regime of the arabian lithosphere controlled mainly by a mantle upwelling and a consequent extensional tectonism along the red sea gulf rift system

joint two dimensional dc resistivity and seismic travel time inversion with cross gradients constraints it is now common practice to perform collocated dc resistivity and seismic refraction surveys that complement each other in the search for more accurate characterization of the subsurface although conventional separate dc resistivity and seismic models can be diagnostic we posit that better results can be derived from jointly estimated models we make the assumption that both methods must be sensing the same underlying geology and have developed an innovative resistivity velocity cross gradients relationship to evaluate the structural features common to both methods the cross gradients function is incorporated as a constraint in a nonlinear least squares problem formulation which is solved using the lagrange multiplier method the resultant iterative two dimensional 2 d joint inversion scheme is successfully applied to synthetic data serving as validation tests here and to field data from collocated dc resistivity and seismic refraction profiling experiments and also compared to conventional separate inversion results the joint inversion results are shown to be superior to those from separate 2 d inversions of the respective data sets since our algorithm leads to resistivity and velocity models with remarkable structural agreement

maximum likelihood estimation of lithospheric flexural rigidity initial loading fraction and load correlation under isotropy topography and gravity are geophysical fields whose joint statistical structure derives from interface loading processes modulated by the underlying mechanics of isostatic and flexural compensation in the shallow lithosphere under this dual statistical mechanistic viewpoint an estimation problem can be formulated where the knowns are topography and gravity and the principal unknown the elastic flexural rigidity of the lithosphere in the guise of an equivalent effective elastic thickness this important geographically varying structural parameter has been the subject of many interpretative studies but precisely how well it is known or how best it can be found from the data abundant nonetheless has remained contentious and unresolved throughout the last few decades of dedicated study the popular methods whereby admittance or coherence both spectral measures of the relation between gravity and topography are inverted for the flexural rigidity have revealed themselves to have insufficient power to independently constrain both it and the additional unknown initial loading fraction and load correlation fac tors respectively solving this extremely ill posed inversion problem leads to non uniqueness and is further complicated by practical considerations such as the choice of regularizing data tapers to render the analysis sufficiently selective both in the spatial and spectral domains here we rewrite the problem in a form amenable to maximum likelihood estimation theory which we show yields unbiased minimum variance estimates of flexural rigidity initial loading frac tion and load correlation each of those separably resolved with little a posteriori correlation between their estimates we are also able to separately characterize the isotropic spectral shape of the initial loading processes

geophysical investigation of lithosphere structure beneath sri lanka using surface and internal load variations this paper describes a method for interpreting the lateral changes in the geological structure of the lithosphere by iteratively calculating the spatial variation of a bottom to top loading ratio we use forsyth s coherence method based on multi taper spectral analysis on overlapping window areas we discuss the geological findings and their comparison with existing geological interpretations for the sri lankan region the low thickness of the elastic plate in the sri lankan region suggests a high geothermal gradient the lithospheric geological structure beneath the sri lankan region is consistent with the main crustal units of sri lanka which belong to different ancient plates together with interpretations based on metamorphic rock units our findings support the existence of thrust contact in the central highlands of sri lanka

elastic thickness determination based on vening meinesz moritz and flexural theories of isostasy elastic thickness te is one of mechanical properties of the earth s lithosphere the lithosphere is assumed to be a thin elastic shell which is bended under the topographic bathymetric and sedi

a geodetic view on isostatic models before the background of more accurate and denser gravity data it is worthwhile to reassess geodetic isostasy currently in geodesy isostatic models are primarily applied to gravity reduction as needed by geoid and gravity modeling the selection of the isostatic model is based on four criteria isostatically reduced gravity anomalies should be 1 geophysically meaningful 2 easy to compute 3 small smooth and therefore easy to interpolate and 4 the indirect effect i e the change of potential and gravity due to isostatic mass replacement should be small in this study we analyze free air anomalies as well as isostatic anomalies based on the airy heiskanen model and on the pratt hayford model in regard to these criteria several facts suggest that free air anomalies are the most realistic type of isostatic anomalies they reflect the actual isostatic compensation are easy to compute and their indirect effect is negligibly small however they are not smooth due to the fact that local topographic loads are only partially compensated smoothness can be achieved by introducing either a mathematical low pass filter or a hydrostatic isostatic model such as the airy heiskanen or the pratt hayford model in both cases the resulting isostatically reduced gravity anomalies fulfill all requirements in order to improve the numerical efficiency a new mathematical description of the pratt hayford model is formulated the level of smoothing with respect to free air anomalies is analyzed in global and regional contexts it turns out that the mechanism of mass compensation in regions of large topographic loads is better described by the airy heiskanen model whereas the pratt hayford model is more suitable for regions of deep ocean trenches

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

the nature of the plate interface and driving force of interseismic deformation in the new zealand plate boundary zone revealed by the continuous gps velocity field 1 new zealand straddles the boundary between the australian and pacific plates cenozoic relative plate motion has resulted in a complex pattern of faulting and block rotation with displacements on individual faults up to hundreds of kilometers however over periods of several years gps measurements show a remarkably smooth pattern of velocities we show here using a new method of back slip analysis that almost the entire plate boundary continuous gps velocity field can be predicted within measurement error from a simple model of elastic distortion due to deep slip on a single plate interface megathrust in the hikurangi and putsegur subduction zones or fault through continental lithosphere beneath the southern alps at the relative plate motion rates this suggests that the main driving force of plate boundary deformation is slip on the deeper moving part of the plate interface without buried creep in localized shear zones beneath individual surface faults the depth at which this deep slip terminates locking point line determines the width of deformation along the hikurangi margin there is also clockwise rotation of 150 km long segment of the fore arc wairoa domain at 4 5 1 ma relative to the australian plate about a pole in western north island model residuals in the velocity field are mainly a result of incomplete averaging of the cycle of slow slip events on the plate interface downdip of the locking point

on the robustness of estimates of mechanical anisotropy in the continental lithosphere a north american case study and global reanalysis lithospheric strength variations both influence and are influenced by many tectonic processes including orogenesis and rifting cycles the long complex and highly anisotropic histories of the continental lithosphere might lead to a natural expectation of widespread mechanical anisotropy anisotropy in the coherence between topography and gravity anomalies is indeed often observed but whether it corresponds to an elastic thickness that is anisotropic remains in question if coherence is used to estimate flexural strength of the lithosphere the null hypothesis of elastic isotropy can only be rejected when there is significant anisotropy in both the coherence and the elastic strengths derived from it and if interference from anisotropy in the data themselves can be plausibly excluded we consider coherence estimates made using multitaper and wavelet methods from which estimates of effective elastic thickness are derived we develop a series of statistical and geophysical tests for anisotropy and specifically evaluate the potential for spurious results with synthetically generated data our primary case study the north american continent does not exhibit meaningful anisotropy in its mechanical strength similarly a global reanalysis of continental gravity and topography using multitaper methods produces only scant evidence for lithospheric flexural anisotropy

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

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

deep structure of the western south african passive margin results of a combined approach of seismic gravity and isostatic investigations abstract the passive margin of the south atlantic shows typical features of a rifted volcanic continental margin encompassing seaward dipping reflectors continental flood basalts and high velocity density lower crust at the continent ocean transition probably emplaced during initial seafloor spreading in the early cretaceous the springbok profile offshore western south africa is a combined transect of reflection and refraction seismic data this paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives the velocity modelling revealed a segmentation of the margin into three distinct parts of continental transitional and oceanic crust as observed at many volcanic margins the lower crust is characterised by a zone of high velocities with up to 7 4 km s the conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities found to be 3100 kg m 3 both approaches identified the body to have a thickness of about 10 km yet the gravity modelling predicted the transition between the high density body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal finally isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced the results imply isostatic equilibrium over large parts of the profile smaller deviations are supposed to be compensated regionally the calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km

increasing long wavelength relief across the southeastern flank of the sierra nevada california abstract a high degree of correlation between present day relative rock uplift measured using continuous gps geodesy and spatially averaged surface elevations suggests that long wavelength topographic relief is presently increasing along the southeastern flank of the sierra nevada range and within an adjacent portion of the northern basin and range province current estimates for erosion rate are an order of magnitude smaller than the relative rates determined by geodesy thus although the uplift serves to enhance long wavelength relief it cannot be explained entirely as an isostatic response to erosion if uplift rates have been constant through time the duration over which the uplift could have been active

elastic thickness structure of south america estimated using wavelets and satellite derived gravity data abstract we used a wavelet formulation of the classical spectral isostatic analysis to invert satellite derived gravity and topography bathymetry for elastic thickness te over south america and its surrounding plates to provide a homogeneous representation of the gravity field for this vast region we corrected free air anomalies derived from a combination of terrestrial marine gravity data with data from the grace and champ satellite missions model eigen cg03c by a simple bouguer slab using a smoothed representation of surface relief wavelengths 125 km the resulting bouguer anomaly compares well with terrestrial data acquired in the central andes and allows te to be confidently estimated for values greater than 10 km the te map resolves regional scale features that are well correlated with known surface structures and shows maximum values of 100 15 km over the archean neoproterozoic core of the continent decreasing to less than 30 km around continental margins several regions of the oceanic plates and continental margins have an elastic thickness less than 10 km we performed a quantitative analysis by comparing the elastic thickness with the thermal structure predicted from the age of oceanic crust and igneous metamorphic rocks this demonstrates that oceanic plates have been weakened by thermal interaction with hotspots and locally by fracturing and hydration near the trench we observe that only the nucleus of the continent has resisted the thermomechanical weakening induced by the rifting of africa and south america along the passive margin and the andean orogeny along the active margin this latter region shows along strike variations in te that correlate with the geotectonic segmentation of the margin and with the pattern of crustal seismicity our results reveal that the rigidity structure follows the segmentation of the seismogenic zone along the subduction fault suggesting a causal relationship that should be investigated in order to improve the understanding and predictability of great earthquakes and tsunamis

the effect of noise on estimates of the elastic thickness of the continental lithosphere by the coherence method 1 we model the lithosphere as a uniform elastic plate overlying an inviscid fluid and loaded with both surface and subsurface fractal loads to generate synthetic topography and gravity data to simulate data having low topographic signal to gravity noise ratio we use an algebraically larger exponent for the subsurface load in the spectral synthesis fractal algorithm the gravity power spectrum then decays less rapidly than that of topography the spectra resembling those for central australia we find that the coherence method using multitaper spectral estimation yields significant underestimates of plate thickness for both low and normal signal to noise ratio data unless the data window is larger than several times the true flexural wavelength we quantify this bias for the parameters used here and apply it as a correction to an effective elastic thickness estimate for central australia obtaining a value of 115 25 km index terms 8110 tectonophysics continental tectonics general 0905 8159 tectonophysics rheology crust and lithosphere 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 citation swain c j and j f kirby the effect of noise on estimates of the elastic thickness of the continental lithosphere by the coherence method geophys res lett 30 11 1574 doi 10 1029 2003gl017070 2003

compensation of swells and plateaus in the north pacific no direct evidence for mantle convection at intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation a low geoid topography ratio 8 m km in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection after subtracting a reference geoid from the observed geoid previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000 4000 km since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid topography ratios 10 20 m km they were believed to reflect the planform of mantle convection we show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10 since the geoid spectrum is red the residual geoid is dominated by degree 11 when the harmonics of the reference geoid are rolled off gradually the regular pattern of geoid highs and lows disappears in the northeast pacific the new residual geoid reflects the lithosphere age offsets across the major fracture zones in the northwest pacific the residual geoid corresponds to isostatically compensated swells and plateaus we have calculated the geoid topography ratio for 10 swells and plateaus and have found a range of compensation depths the highest geoidaopography ratio of 5 5 m km occurs on the flanks of the hawaiian swell intermediate ratios occur in four areas including the midway swell these intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of airy compensated seamounts low geoid topography ratios occur over the remaining five areas e g emperor seamounts reflecting the absence of a thermal swell our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid we see only indirect evidence of thermal plumes reheating the lower lithosphere

effects of loads in the upper crust on estimates of the elastic thickness of the lithosphere summary the admittance function and coherence relating gravity and topography are important constraints on the elastic thickness te of the lithosphere forsyth 1985 modelled them with a lithosphere loaded both externally by topography and internally by density contrasts such as initial undulations on the moho his approach has yielded thicknesses as high as 150 km for the continental lithosphere mckenzie fairhead 1997 argued that this method can only supply an upper bound on te because the response is biased by noise due to density variations in the upper crust we have used forsyth s approach to analyse the effect of shallow heterogeneities such as sedimentary basins and igneous intrusions simulating them by a thin layer with a laterally variable density the information content of the admittance and coherence is concentrated in the wavenumbers at which the two responses roll off to half their zero wavenumber values these two parameters are used to explore the interdependence of the retrieved elastic thickness and internal external loading ratio f provided the internal load is less than 10 per cent of the external it does indeed bias the response so that te is overestimated however for predominantly internal loads the reverse is the case and the value of te obtained by assuming equal internal and external loads is actually a lower bound it is particularly important to allow for the flexure due to upper crustal loads when the lithosphere is old and topography subdued due to prolonged erosion application of our approach to mckenzie fairhead s 1997 western australia data shows that the loading ratio depends strongly on wavenumber but that it is still possible to use the response data to constrain te which must be at least 90 km if the results obtained by conventional spectral estimation are accepted at least 40 km using the multitaper method

elastic thickness of the himalayan tibetan orogen estimated from the fan wavelet coherence method and its implications for lithospheric structure abstract this paper is concerned with the spatial variation of the elastic thickness t e in the himalayan tibetan orogen and its implications for the understanding of lithospheric structure and deformation new t e maps of the himalayan tibetan orogen are generated from the new combined satellite terrestrial gravity eigen6c3stat and topography etopo1 models using the fan wavelet coherence technique the high t e values 60 km are observed in the indian shield and the tarim basin while the low t e values 40 km are prevailing over most of the tibetan plateau we also find that the t e distributions estimated using the high and low central wavenumber i e k 0 wavelets are different in southern tibet by combining constraints from the current seismic and thermo rheological modeling this difference may reflect that the strength of the lithosphere in southern tibet mainly resides in the lithospheric mantle within most parts of central and northern tibet the t e values are relatively low 30 km indicating that the entire lithosphere is weak our analysis finds that the t e variations are probably associated with changes in lithosphere structure we interpret the abrupt changes in t e between southern and northern tibet to indicate the location of the underthrusting strong indian lithosphere beneath tibet our results suggest that the northern limit of the underthrusting indian plate reaches as far north as the hindu kush and the southwestern margin of the tarim basin near 70 80 e and extends along the bangong nujiang suture in the central 80 87 e eastward between 87 e 93 e it may move to the indus tsangpo suture indicating a delamination of indian mantle lithosphere there further to the east it may underthrust northward beneath the eastern qiangtang block

variations of the effective elastic thickness over the ross sea and transantarctic mountains and implications for their structure and tectonics abstract the effective elastic thickness t e is a proxy for lithospheric strength and it depends primarily on the thermal gradient and composition of the lithosphere accordingly spatial variations in t e reflect changes in lithospheric properties and can be used to better understand the structure and tectonics of particular regions in this paper we investigate the ross sea and transantarctic mountains in terms of t e using gravity and topographic data and the fan wavelet transform technique the results reveal that relatively high t e values dominate in the extensional basins of the ross sea and the hinterland of transantarctic mountains whereas very low t e values occur along the transantarctic mountain front and in the deep ocean basin with the lowest t e values are found the vicinity of ross island and onshore in northern victoria land in addition the spatial variations in t e correlate well with lithospheric structure at the regional scale by combining these findings with published seismic and heat flow data we conclude that the presence of a zone of anomalously low t e values parallel to the coast indicates that the lithosphere beneath the transantarctic mountain front is extremely weak due to cenozoic volcanism and extension the t e values increase from the transantarctic mountain front 7 km toward the center of the continent 80 km which indicates that the continental lithosphere underlying east antarctica belongs to the classic gondwanan craton the increase in t e indicates that the transantarctic mountain front marks the continent continent boundary between east antarctica and west antarctica the t e values in the other extensional basins of the ross sea exhibit little variation and average approximately 35 km the relatively high t e values are interpreted to indicate that the lithosphere cooled and became mechanically stronger between late cretaceous extension and eocene neogene deposition

an effective elastic thickness map of australia from wavelet transforms of gravity and topography using forsyth s method 1 we have developed a wavelet version of forsyth s coherence method for estimating the effective elastic thickness te of the lithosphere from gravity and topography we have previously shown that the fan wavelet is particularly well suited to making localised coherence estimates and here we progress the method by showing how the coherence can be modelled by using the wavelet transforms of gravity and topography to infer the initial loads that flex the plate the effectiveness of the method is demonstrated on synthetic data from a model having a rectangular area of large te embedded in a uniform low te surrounding and a load ratio that increases with wave number the results for the australia region show a core of high te under north and central australia with much lower values to the east and also unexpectedly under the western parts of the yilgarn and pilbara cratons

flexural rigidity of the basin and range colorado plateau rocky mountain transition from coherence analysis of gravity and topography stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography however the spatial resolution of coherence analysis has been limited by use of two dimensional periodogram spectra from very large 105 km 2 windows that generally include multiple tectonic features using a two dimensional spectral estimator based on the maximum entropy method the spatial resolution of flexural properties can be enhanced by a factor of 4 or more enabling more detailed analysis at the scale of individual tectonic features this new approach is used to map the spatial variation of flexural rigidity along the basin and range transition to the colorado plateau and middle rocky mountains physiographic provinces large variations in flexural isostatic re sponse are found with rigidities ranging from as low as 8 7x102o n m elastic thickness t 4 6 km in the basin and range to as high as 4 1x1024 n m t 77 km in the middle rocky mountains these results compare favorably with independent determinations of flexural rigidity in the region areas of low flexural rigidity correlate strongly with areas of high surface heat flow as is expected from the contingence of flexural rigidity on a temperature dependent flow law also late cenozoic normal faults with large displacements are found primarily in areas of low flexural rigidity while deformation fronts of mesozoic tertiary overthrusts occur 0 to 100 km east of the low rigidity region the highest flexural rigidity is found within the archean wyoming craton where evidence suggests that deeply rooted cratonic lithosphere may play a role in deter mining the distribution of tectonism at the surface

effective elastic thickness of south america and its implications for intracontinental deformation 1 the flexural rigidity or effective elastic thickness of the lithosphere te primarily depends on its thermal gradient and composition consequently maps of the lateral variability of te in continents reflect their lithospheric structure we present here a new te map of south america generated using a compilation of satellite derived grace and champ missions and terrestrial gravity data including egm96 and sagp and a multitaper bouguer coherence technique our te maps correlate remarkably well with other proxies for lithospheric structure areas with high te have in general high lithospheric mantle shear wave velocity and low heat flow and vice versa in this paper we focus on the te of the stable platform we find that old cratonic nuclei mainly archean and early middle proterozoic have in general high te 70 km while the younger patagonian phanerozoic terrane has much lower te 20 30 km suggesting that te is related to terrane age as has already been noted in europe within cratonic south america te variations are observed at regional scale relatively lower te occurs at sites that have been repeatedly reactivated throughout geological history as major sutures rift zones and sites of hot spot magmatism today these low te areas are surrounded by large cratonic nuclei they concentrate most of the intracontinental seismicity and exhibit relatively high surface heat flow and low seismic velocity at 100 km depth this implies that intracontinental deformation focuses within relatively thin hot and hence weak lithosphere that cratonic interiors are strong enough to inhibit tectonism and that the differences in lithospheric rigidity structure and composition between stable cratons and sites of intracontinental deformation are not transient and may have been maintained in some cases for at least 500 m y

gravity anomalies crustal structure and flexure of the lithosphere at the baltimore canyon trough seismic and gravity anomaly data have been used to examine the long term mechanical properties of the lithosphere beneath the baltimore canyon trough a 15 km deep sedimentary basin in the rifted u s atlantic margin seismic data constrain the crustal structure at the time of rifting as well as the amount of sedimentation and erosion that has occurred the gravity effect of the initial crustal structure sedimentary loading and erosional unloading have been computed and compared to the observed free air gravity anomaly the best fitting model is one in which the elastic thickness of the lithosphere te changes at the east coast magnetic anomaly landward of the anomaly te is significantly smaller than previously reported values for continental or oceanic lithosphere of the same thermal age seaward of the anomaly however te follows the depth to the 450 c oceanic isotherm which also describes the response of oceanic lithosphere to seamount and oceanic island loads these results are interpreted as indicating that the shelf part of the trough is underlain by continental lithosphere which following rifting did not acquire any significant long term strength

on the recovery of effective elastic thickness using spectral methods examples from synthetic data and from the fennoscandian shield there is considerable controversy regarding the long term strength of continents te while some authors obtain both low and high te estimates from the bouguer coherence and suggest that both crust and mantle contribute to lithospheric strength others obtain estimates of only 25 km using the free air admittance and suggest that the mantle is weak at the root of this controversy is how accurately te can be recovered from coherence and admittance we investigate this question by using synthetic topography and gravity anomaly data for which te is known we show that the discrepancies stem from comparison of theoretical curves to multitaper power spectral estimates of free air admittance we reformulate the admittance method and show that it can recover synthetic te estimates similar to those recovered using coherence in light of these results we estimate te in fennoscandia and obtain similar results using both techniques te is 20 40 km in the caledonides 40 60 km in the swedish svecofennides 40 60 km in the kola peninsula and 70 100 km in southern karelia and svecofennian central finland independent rheological modeling using a xenolith controlled geotherm predicts similar high te in central finland because te exceeds crustal thickness in this area the mantle must contribute significantly to the total strength te in fennoscandia increases with tectonic age seismic lithosphere thickness and decreasing heat flow and low te correlates with frequent seismicity however in proterozoic and archean lithosphere the relationship of te to age is ambiguous suggesting that compositional variations may influence the strength of continents copyright 2004 by the american geophysical union

physical and chemical controls on the composition of waters in sedimentary basins abstract the composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion the south west louisiana gulf coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section the simultaneous systematic variation in dissolved na k mg ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate carbonate mineral assemblages is a first order control on fluid compositions the chemical potential of chloride or alternatively total anionic charge appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region this variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper mudstone dominated sediments where the composition of the fluid is largely rock buffered the ultimate origin of the fluid and its pathway of chemical evolution may be obscured at least in terms of major solute composition non buffered components such as cl and br or isotopic compositions are more likely to retain information on original end member fluid compositions and reaction pathways

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

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

an assessment of the accuracy of admittance and coherence estimates using synthetic data previous work has shown that estimates of the admittance between topography and free air gravity anomalies are often biased by spectral leakage even after the application of multiple prolate spheroidal wavefunction data tapers despite this a number of authors who have used the free air admittance method to estimate the weighted average effective elastic thickness of the lithosphere te and to identify topography supported by mantle convection have not tested their methods using synthetic data with a known relationship between topography and gravity in this paper i perform a range of such tests using both synthetic surface data and synthetic line of sight los accelerations of satellites orbiting around an extra terrestrial planet it is found that spectral leakage can cause the estimated admittance and coherence to be significantly in error but only if the box in which they are estimated is too small the definition of small depends on the redness of the gravity spectrum there is minimal error in the whole box weighted average estimate of te if the admittance between surface gravity and topography is estimated within a box at least 3000 km wide when the synthetic uniform te is less than 20 km and the coherence is high the errors in te are mostly 5 km for all box sizes greater than 1000 km on the other hand when the true te is greater than 20 km and the box size is 1000 km the best fitting te is likely to be at least 5 10 km less than the true te however even when the coherence is high it is not possible to use elastic plate admittance models to distinguish between real and spurious small fractions of internal loading when the boxes are smaller than 2000 km in width noise in the gravity introduces error and uncertainty but no additional bias into the estimates of the admittance it does however bias estimates of te calculated using the coherence between bouguer gravity and topography the admittance at wavelengths between 1000 and 4000 km which is of interest when studying mantle convection is only well resolved when the box size is larger than 2000 km or the synthetic te is less than 15 km and the true long wavelength admittance is greater than 30 mgal km 1 when the box size is 1000 km it is not possible to distinguish between real and spurious convective support of long wavelength topography at the altitude of most orbiting satellites 200 400 km gravity is much redder than it is at ground level therefore it is necessary to use much larger boxes when using los accelerations to estimate the admittance than it is when using surface measurements however gradual changes in the altitude of the satellite or even occasional sudden changes make almost no difference to the estimate of the admittance finally three short investigations are undertaken concerning the effect on the admittance of the application of irregular windows to the data of lateral variations in te and of the infilling of flexural moats by low density sediment it is found that the bias due to the application of an irregular window is not significantly greater than the bias due to the application of a regular window of similar size that the best fitting te is strongly biased by the value associated with the highest amplitude topography and that sediment infill raises the admittance at wavelengths corresponding to the width of the moat

elastic thickness estimates for the northern lowlands of mars abstract the northern lowlands cover approximately one third of mars surface and are a fundamental part of the geologic evolution of mars we examine the admittance signature the ratio of gravity to topography data in the spectral domain for four regions in the northern lowlands that are both well resolved in the gravity data and have well constrained lithospheric parameters these regions also have topographic power spectra similar to many highlands regions a cartesian multitaper method is used to calculate admittance we compare the observed admittance signatures to those predicted from models of lithospheric flexure on the basis of these comparisons we estimate the thickness of the martian elastic lithosphere t e required to support the observed topographic load since the time of loading we use both top and bottom loading models to derive values of elastic thickness and crustal thickness or apparent depth of compensation although all four regions are best fit by bottom loading models we obtain best fit elastic thickness estimates between 10 and 25 km with an acceptable error range of 0 to 45 km these low elastic thickness estimates are consistent with the formation in the noachian when heat flow was high the consistency in t e estimates between the noachian highlands and lowlands basement suggests that both regions of the crust formed at similar times the paucity of crustal magnetization in the lowlands is thus more likely a result of demagnetization than formation following shut down of the dynamo

a process oriented approach to modeling the gravity signature of continental margins the gravity anomaly at a continental margin can be regarded as the result of all the processes that have shaped it through time these include rifting sedimentation and magmatic underplating this article discusses a new process oriented approach to gravity interpretation which uses seismic constraints on sediment thickness and the underplate at continental margins to estimate the gravity anomaly that is associated with rifting sedimentation and magmatic underplating by comparing the calculations to the observed free air gravity edge effect anomaly we have been able to determine the elastic thickness and crustal structure and to provide an estimate of the location of the ocean continent transition since crustal structure is a proxy for the amount of heating that occurred at the time of rifting the approach has important implications for the deep water hydrocarbons industry

temperature control of continental lithosphere elastic thickness te vs vs article i nfo the elastic thickness of the continental lithosphere is closely related to its total strength and therefore to its susceptibility to tectonic deformation and earthquakes recently it has been questioned whether the lithosphere thickness and strength are dependent on crustal and upper mantle temperatures and compositions as predicted by laboratory data we test this dependence regionally by comparison in northwestern north america of the effective elastic thickness te from topography gravity coherence with upper mantle temperatures mapped by shear wave tomography velocities vs and other temperature indicators the te values are strongly bimodal as found globally less than 20 km for the hot cordillera backarc and over 60 km for the cold stable craton these te correspond to low vs beneath the cordillera and high vs beneath the craton model temperature depth profiles are used to estimate model te for comparison with those observed only limited areas of intermediate thermal regimes i e thermotectonic ages of 300 ma have intermediate te that suggest a weak lower crust over a stronger upper mantle there are large uncertainties in model te associated with composition water content strain rate and decoupling stress threshold however with reasonable parameters model yield stress envelopes correspond to observed te for thermal regimes with 800 900 c at the cordillera moho and 400 500 c for the shield in agreement with temperatures from vs and other estimators our study supports the conclusion that lithosphere elastic thickness and strength are controlled primarily by temperature and that laboratory based rheology generally provides a good estimate of the deformation behaviour of the crust and upper mantle on geological time scales

lithospheric domains and controls on kimberlite emplacement slave province canada evidence from elastic thickness and upper mantle composition 1 we have mapped the deep structure of the slave craton by combining analysis the effective elastic thickness te with data on mantle samples from numerous kimberlites three dimensional mapping of the subcontinental lithospheric mantle sclm using mantle derived xenoliths and xenocrysts in kimberlites has shown that much of the craton is underlain by a strongly layered sclm a highly depleted upper layer low in basaltic components ca al fe is separated from a relatively fertile lower layer by a sharp boundary this boundary lies at 140 150 km depth in the lac de gras area and shallows to 100 km in the northern and southern parts of the craton weak lithosphere te 56 km in the younger eastern part of the craton is separated from the older western part by a zone of steep te gradient parallel to the major locus of kimberlite intrusion which may map the deep extension of the boundary between the two domains another strong te gradient across the kilohigok basin accompanies a marked compositional change in the upper layer of the lithospheric mantle the basin probably marks a major translithospheric fault correlations between te and mantle composition suggest that te is strongly influenced by the rheology of the upper mantle

behavior of the brittle crust in wide plate boundary zones in wide and active continental plate boundary zones ductile flow in the deeper and strong parts of the lithosphere may control crustal deformation this is likely if average resistive shear stresses on faults in the brittle crust are much less than 108 pa and the underlying bulk effective viscosity is much greater than 1021 pa s in this case a simple model of distributed deformation referred to as the floating block model may be useful this treats the crust as an array of rotating and translating rigid blocks which are floating on an underlying continuous flow with a constant rheology the model is analyzed in detail in this paper because it has the potential to link detailed observations of crustal deformation with the large scale flow crustal blocks are defined by at least two sets of faults the kinematics of crustal deformation can be described in terms of the motions of these blocks both the relative motion on block boundaries faults and block tilting about a horizontal axis can be described in terms of the underlying flow and block rotation about a vertical axis however rotations about a vertical axis which are an important component of the crustal deformation will depend not only on the underlying flow but also on the shape orientation and arrangement of the crustal blocks the average rotation rate about a vertical axis over finite rotations will be significantly different from that predicted at any instant also the rotation history is considerably complicated if as is likely the underlying flow field or block shape has changed with time these aspects of crustal deformation are discussed with reference to real zones of active deformation in new zealand greece and western north america

is it all in the crust new observations suggest that earthquakes on land are only skin deep confined to the earth s outermost layer of crust this has prompted a rethink of what gives the tectonic plates their strength

shear margins continent ocean transform and fracture zone boundaries as exploration extends into deeper water it has become more important to understand the nature of the earth s crust beneath offshore sediments that is heat flow and related source rock potential usually require that sediments be deposited over continental crust hence delineating the boundary between oceanic and continental crust becomes important the material in this article was originally presented at the workshop the crust and its structure at seg s recent annual meeting in calgary these results part of a larger dissertation research project also have been presented to several oil and gas exploration companies in houston

a reassessment of spectral te estimation in continental interiors the case of north america 1 conventional spectral te studies use the real part of the admittance between gravity anomalies and topography or alternatively the square of the magnitude of the coherency i e coherence here we show the utility of treating both the admittance and coherency as complex quantities inverting the real parts to estimate te we use the imaginary parts to tell if the inversion is biased by noise one method inverts the square of the real coherency with the internal to total load ratio f derived as a function of wave number directly from the gravity and topography the other method inverts the real part of the admittance assuming that f is wave number independent we test the methods using synthetic elastic plate models loaded at the surface and moho in such a way that the final relief is the actual north american topography in some of the models we add gravity noise generated by a model having both surface and internal loads such that the final topography is zero and find that both methods are susceptible to noise application of the two methods to north america gives te maps showing substantial agreement except in regions affected by noise but these are not a dominant part of the total area given the suggested mechanisms by which noise might arise it is not surprising that it is not a more widespread feature of the north american craton importantly both methods show that large parts of the canadian shield are characterized by te 100 km

why no anomaly is visible over most of the continent ocean boundary in the global crustal magnetic field abstract it is generally believed that an anomaly should be expected over the continent ocean c o boundary in the global magnetic anomaly maps however such maps prepared at satellite altitude do not show an anomaly over most of the c o boundary we address this issue by a forward modelling technique we use a geographic information system gis technique to integrate information of the various geological units of continents and oceans the seismic crustal structure and standard susceptibility values of the rock types and compute a global vertically integrated susceptibility vis model in addition a remanent magnetisation model is used for the oceanic crust combining the induced and remanent magnetisation models the crustal field anomaly is predicted at a satellite altitude of 400 km for spherical harmonic degrees 16 80 the results show that an anomaly is indeed not predicted over the c o boundaries where the oceanic crust is flanked by continental regions phanerozoic in age we demonstrate that largely a c o anomaly over these regions is not even expected in the global magnetic anomaly map for degrees 1 80 confirming that it is not a long wavelength feature masked by the core field an anomaly however is predicted over some parts of the c o boundary where the flanking geological provinces over the continents are precambrian in age we argue that the vis values for the continents and oceans are largely comparable and therefore no anomaly is expected over most of the c o boundary

mapping the mechanical anisotropy of the lithosphere using a 2d wavelet coherence and its application to australia abstract we develop a new method for imaging the spatial variations of the anisotropy of the flexural response of the lithosphere and apply it to recent topographic and gravity data sets over australia the method uses two dimensional morlet wavelet transforms superposed in a strictly controlled geometry to estimate the auto and cross spectra of the two data sets in a number of different directions the resulting wavelet coherence is a function of scale or wavelength as well as orientation and is inverted at each spatial location for the three parameters of an anisotropic thin elastic plate model i e maximum and minimum flexural rigidities and the orientation of the maximum extensive tests of the method on synthetic anisotropic but uniform data sets show that it retrieves the amplitude and orientation of the anisotropy with useful accuracy the results for australia west of 143 e show a strong correlation with the shallower layers 75 175 km of a recent model of seismic sv wave azimuthal anisotropy the weak axes i e of minimum flexural rigidity in most cases are approximately at right angles to the fast axes of the seismic anisotropy implying that for precambrian australia they arise from the same source this is most likely deformation resulting from the most recent episode of orogeny

spatial variations of the effective elastic thickness te using multitaper spectral estimation and wavelet methods examples from synthetic data and application to south america 1 improved recovery of the spatial variability in the effective elastic thickness te of the lithosphere is important to interpret earth s structure and geodynamics here we use synthetic topography and gravity data to systematically compare the recovery of te using the multitaper windowing scheme of perez gussinye et al 2004 2007 with the wavelet approach of kirby and swain 2008 we find that spuriously high te estimates previously present in synthetic tests with the multitaper are artifacts arising from fourier transform edge effects introduced by splitting the data into land and ocean areas prior to load deconvolution hence we adopt the mixed land ocean loading scheme of kirby and swain 2008 using this approach we find that underestimation of te when the transitional wavelength approaches the window size is largely related to the limited size of the analysis window and not to random correlations of initial loading surfaces as occurs in the wavelet method to attenuate this bias we apply a correction factor to our resulting multitapered te and find that this technique more accurately recovers steep te gradients and small scale te anomalies than the wavelet method finally we recalculate te of south america using the multitaper and the mixed land ocean approach the new te distribution is very similar to that of perez gussinye et al 2007 2008 suggesting that fourier transform edge effects are less severe in the real earth than in synthetic data this difference arises because coastlines are much less rough in the real earth than in our synthetic data on the basis of the synthetic results we interpret that most of the small scale features present in the multitaper but not in wavelet te estimates of south america presented by tassara et al 2007 are real these include high te over the andean flat subduction zones and low te along the dyke swarms of the parana flood basalts the tacutu graben and amazonian basin

a re assessment of focal depth distributions in southern iran the tien shan and northern india do earthquakes really occur in the continental mantle summary we investigate the depth distribution of earthquakes within the continental lithosphere of southern iran the tien shan and northern india by using synthetic seismograms to analyse p and sh body waveforms in the zagros mountains of southern iran earthquakes are apparently restricted to the upper crust depths of 20 km whereas in the tien shan and northern india they occur throughout the thickness of the continental crust to depths of 40 45 km we find no convincing evidence for earthquakes in the continental mantle of these regions in spite of previous suggestions to the contrary and question whether seismicity in the continental mantle is important in any part of the world in some regions such as iran the aegean tibet and california seismicity is virtually restricted to the upper continental crust whereas in others including parts of east africa the tien shan and northern india the lower crust is also seismically active although usually less so than the upper crust such variations cannot reliably be demonstrated from published catalogue or bulletin locations even from ones in which depth resolution is generally improved in contrast to the oceanic mantle lithosphere in which earthquakes certainly occur the continental mantle lithosphere is we suggest virtually aseismic and may not be significantly stronger than the lower continental crust these variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness suggesting that the strength of the continental lithosphere resides in the crust and require some modification to prevalent views of lithosphere rheology

dynamics of crustal compensation and its influences on crustal isostasy deviation from isostasy is commonly believed to be caused by the strength of the earth s lithosphere an analysis of crustal compensation dynamics suggests that the deviation may have a dynamic origin the analysis is based on analytic models that assume that 1 the medium is incompressible and has a layered and linear viscoelastic rheology and 2 the amplitude of topography is small compared with its wavelength the models can describe topographic relaxation of different density interfaces at both small e g postglacial rebound and large time scales the models show that for a simple crust mantle system with topography at the earth s surface and moho representing the only mass anomalies while the crust always approaches the isostatic state at long wavelengths 800 km crustal isostasy may not be an asymptotic limit at short wavelengths depending on crustal and lithospheric rheology for a crust with viscosity smaller than lithospheric viscosity at wavelengths comparable with widths of orogenic belts i e 300 km the crust tends to approach a state with significant overcompensation i e excess topography at the moho within a timescale of about 107 years and this characteristic time depends on wavelengths and crustal viscosity this overcompensation is greater for weaker crust and stronger lithosphere a thicker crust or lithosphere also enhances this overcompensation if crustal and lithospheric viscosities are both large and comparable the asymptotic state for the crust displays a slight undercompensation for an elastic and rigid upper crust the crust eventually becomes undercompensated after a characteristic decay time of topography at the moho the characteristic time is dependent on viscosity and thickness of the lower crust the deviation from isostasy arises because these viscosity structures result in a ratio of vertical velocity at the surface to vertical velocity at the moho which in the asymptotic state for short wavelengths differs from the ratio of density contrast at the moho to that at the surface

variations in elastic thickness in the canadian shield we have compared different spectral methods the standard windowed fourier transform the multitaper method mtm and the maximum entropy method mem to determine the coherence between bouguer gravity and topography and estimate the variations of the effective elastic thickness te of the lithosphere these methods differ significantly in spatial resolution 1000 to 2000 km determined by the width of the windows required to resolve the wavelength of transition from low to high coherence the tests with synthetic data show that the standard deviation on elastic thickness estimates is high with the three methods but that the mean value obtained by the maximum entropy is close to the input value while the other methods underestimate the elastic thickness in the canadian shield the effective elastic thickness values vary from c40 km to n100 km over distances less than the width of the moving window there is no clear geographical or geological pattern in these variations as high and low elastic thickness values are found near the edge as well as in the center of the continent and in all provinces regardless of their age correlation between te and the heat flow field is weak the values of te being much higher beneath hudson bay than the williston basin are consistent with models of basin evolution and seismic estimates of mantle temperatures d 2004 elsevier b v all rights reserved

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

lithospheric structure of venus from gravity and topography abstract there are many fundamental and unanswered questions on the structure and evolution of the venusian lithosphere which are key issues for understanding venus in the context of the origin and evolution of the terrestrial planets here we investigate the lithospheric structure of venus by calculating its crustal and effective elastic thicknesses tc and te respectively from an analysis of gravity and topography in order to improve our knowledge of the large scale and long term mechanical behaviour of its lithosphere we find that the venusian crust is usually 20 25 km thick with thicker crust under the highlands our effective elastic thickness values range between 14 km corresponding to the minimum resolvable te value and 94 km but are dominated by low to moderate values te variations deduced from our model could represent regional variations in the cooling history of the lithosphere and or mantle processes with limited surface manifestation the crustal plateaus are near isostatically compensated consistent with a thin elastic lithosphere showing a thickened crust beneath them whereas the lowlands exhibit higher te values maybe indicating a cooler lithosphere than that when the venusian highlands were emplaced the large volcanic rises show a complex signature with a broad range of te and internal load fraction f values finally our results also reveal a significant contribution of the upper mantle to the strength of the lithosphere in many regions

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

structural softening of the lithosphere structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant structural softening is fundamentally different than material softening where the decrease in stress is generated by a change in rheological properties with progressive deformation such as grain size reduction resulting from large shearing strain we study structural softening generated by folding of the crust mantle boundary which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere for ductile rheologies the stress decrease represents a decrease of the effective lithospheric viscosity which is proportional to the ratio of stress to lithospheric shortening strain rate we present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate the decrease in effective viscosity can be up to 10 fold

isostatic response of the australian lithosphere estimation of effective elastic thickness and anisotropy using multitaper spectral analysis gravity and topography provide important insights regarding the degree and mechanisms of isostatic compensation the azimuthally isotropic coherence function be tween the bouguer gravity anomaly and topography evolves from high to low for increasing wavenumber a diagnostic that can be predicted for a variety of lithospheric loading models and used in inversions for flexural rigidity thereof in this study we investigate the isostatic response of continental australia we consider the effects of directionally anisotropic plate strength on the coherelce the anisotropic coherence function is calculated for regions of australia that have distinctive geological and geophysical properties the coherence estimation is performed by the thomson multiple slepian taper spectral analysis method extended to two dimensional fields our analysis reveals the existence of flexural anisotropy in central australia indicative of a weaker n s direction of lower te this observation is consistent with the suggestion that the parallel faults in that area act to make the lithosphere weaker in the direction perpendicular to them it can also be related to the n s direction of maximum stress and possibly the presence of e w running zones weakened due to differential sediment burial rates we also demonstrate that the multitaper method has distinct advantages for computing the isotropic coherence function the ability to make many independent estimates of the isostatic response that are minimally affected by spectral leakage results in a coherence that is more robust than with modified periodogram methods particularly at low wavenumbers our analysis elucidates the reasons for discrepancies in previous estimates of effective elastic thickness te of the australian lithosphere in isotropic inversions for te we obtain values that are as much as a factor of 2 less than those obtained in standard inversions of the periodogram coherence using bouguer gravity and topography but greater than those obtained by inversions that utilize free air rather than bouguer gravity and ignore the presence of subsurface loads however owing to the low spectral power of the australian topography the uncertainty on any estimate of te is substantial

wavelet analysis of the coherence between bouguer gravity and topography application to the elastic thickness anisotropy in the canadian shield summary we use a wavelet transform to compute the local and azimuthal variations of the coherence between bouguer gravity and topography in eastern canada the isotropic coherence is calculated by averaging the wavelet spectra from optimally overlapping 2 d morlet wavelets having an isotropic spectral envelope in adjacent directions within 180 defining the so called fan wavelet the isotropic wavelet coherence spectrum is inverted to obtain local estimates of the elastic thickness te of the lithosphere we calculate the anisotropic coherence by restricting the fan wavelet over an azimuthal range of 90 the direction of maximum coherence is diagnostic of the direction of preferred isostatic compensation or the direction where the lithosphere is weakest the coherence is inverted using the theoretical response of a thin anisotropic plate model we have carried out extensive tests on synthetic topography and bouguer gravity data sets to verify that 1 the wavelet method can recover te for simple models with either homogeneous or spatially variable rigidity patterns and that 2 the method can determine azimuthal variations in the 2 d coherence for homogeneous models with anisotropic te we have used data from the eastern canadian shield to infer the variations in te and the anisotropy of the coherence the relative variations in te show trends similar to those obtained in previous studies that used different spectral methods the wavelet transform gives te values between 30 and 120 km te is generally high 80 km throughout eastern canada lower values 30 60 km are found in the eastern grenville province in the northern appalachians and in the superior province in the great lakes region the high values found in hudson bay are consistent with previous studies of elastic thickness and models of basin subsidence the direction of maximum coherence obtained from the wavelet method is also consistent with our previous results obtained with the multitaper method and shows that the weak mechanical axis is perpendicular to the fast seismic axis where seismic anisotropy has been detected

elastic thickness determination from on orbit goce data and crust1 0 elastic thickness te is a parameter representing the lithospheric strength with respect to the loading those places having large values of elastic thickness flexes less in this paper the on orbit measured gravitational gradients of the gravity field and steady state ocean circulation explorer goce mission are used for determining the elastic thickness over africa a forward computational method is developed based on the vening meinesz moritz vmm and flexural theories of isostasy to find a mathematical relation between the second order derivative of the earth s gravity field measured by the goce satellite and mechanical properties of the lithosphere the loading of topography and bathymetry sediments and crystalline masses are computed from crust1 0 in addition to estimates of laterally variable density of the upper mantle young s modulus and poisson s ratio the second order radial derivatives of the gravitational potential are synthesised from the crustal model and different a priori values of elastic thickness to find which one matches the goce on orbit gradient this method is developed in terms of spherical harmonics and performed at any point along the goce orbit without using any planar approximation our map of te over africa shows that the intra continental hotspots and volcanoes such as ahaggar tibesti darfur cameroon volcanic line and libya are connected by corridors of low te the high values of te are mainly associated with the cratonic areas of congo chad and the western african basin

lithospheric thickness and mechanical strength of the indian shield abstract we determined the relative mechanical strength of the indian subcontinent by a robust coherence method based on multitaper spectral analysis on overlapping windows of equal size representing the elastic properties of the lithosphere by a transitional coherence wavelength we bypass the need to input rheological variables such as young s modulus or poisson s ratio which have been shown to vary on other continents we compare our measurements in terms of age and find that regions underlain by proterozoic basement are on average stronger than regions of archaean age this is in stark contrast to the common assumption that mechanical strength should increase progressively with time this suggests they are indeed mechanically distinct and that the transitional coherence wavelength adequately captures this distinction between regions of relative strength difference conversions of these values to absolute effective elastic thickness show a greater overlap which is due to the many strong assumptions required in obtaining such absolute values and argues against their usage as a robust and well constrained property of the elastic lithosphere

hydrochemical isotopic tendencies to define hydraulic mobility of formation water at the samaria sitio grande oil field mexico abstract the chemical major elements and isotopic 2 h 3 h 13 c 18 o composition of formation water is presented for the sitio grande oilfield se mexico extracted from 28 production wells from the carbonate reservoir at a depth between 3585 and 4545 m b s l the linear 18 o d trend explains the formation of reservoir water as part of three subsequent stages a the evaporation of marine water at the surface causing enrichment of both 18 o and d values b a reversal trend with decreasing 18 o and d ratios by the extreme evaporation of brines and c the subsequent mixing with isotopically depleted meteoric water a sw ne directed flow direction of deep groundwater systems is indicated by parallel oriented isoline trends of stable isotope ratios and conservative elements supported by the dominance of parallel directed microfractures and extensional faults and by tracer test results the arrival of artificial tritium three years after tracer injection in the well sg 85 reflects a the importance of long term monitoring of tracer tests and b an estimated flow velocity of 2 2 10 5 m s as the arrival of the tracer was detected exclusively in three production wells towards the sw of the injection site lateral migration of groundwater occurs mainly along defined channel like conducts with a narrow dispersion angle of 39 surface water injected as part of an enhanced oil recovery program was not detected in the production zone by chemical and isotopic methods this fact is not a proof for lacking hydraulic conductivity as the large reservoir size and the termination of the injection program in 1996 could have caused a complete dilution of the injected fluids restricted vertical flow movement between reservoir layers and insignificant temperature dependent reaction processes is indicated by the heterogeneous distribution of 18 o and cl values throughout the groundwater column of most reservoir zones little influence of water rock interaction such as ca mg exchange processes between formation water and host rock is reflected by the heterogeneous abundance of calcium in the liquid phase in both dolomite free and dolomitized carbonate host rock layers

flexural models of continental lithosphere based on the long term erosional decay of topography summary the most easily observed geophysical characteristic of continents is their surface topography topographic power in the spectral range 1000 100 km has been computed for north american geological provinces and is found to decrease systematically as the tectonic age of the underlying crust increases these data have been quantitatively modelled in terms of topographic erosion and stress relaxation within the continental lithosphere through geological time spans the lithosphere is treated as a thin maxwell viscoelastic plate which deforms flexurally in response to the vertical surface loads of topography and its erosion effective rheological model parameters are flexural rigidity d a measure of the lithosphere s elastic strength and viscous relaxation time constant a measure of its viscosity erosion is assumed to occur at a rate linearly proportional to the height of topography at any given time the proportionality being determined by the erosion time constant the models are constructed in space and time frequency domains using simple linear systems theory two loading schemes are considered model 1 assumes that a flexurally competent lithosphere is suddenly loaded by topography which subsequently erodes whereas model 2 ignores the emplacement and erosion of topography during orogenesis when the lithosphere is thermally activated and considers only the load effected by erosion beginning some time later when the lithosphere has cooled and strengthened both models predict that the remnant topography of very old regions such as the canadian shield can be maintained in fact is favoured by a viscoelastic lithosphere with as small as 1 10ma the continued existence of such topography does not necessarily imply significant long term lithospheric strength model 1 results indicate low values of the flexural rigidity d 1021 1022nm which are interpreted to characterize not so much presentday d but that which was effective when topography was formed model 2 more successfully reproduces the data and constrains the product d to values 1024 1025nm ma which compare very favourably to those found from studies of individual loads on continental lithosphere the model results show that the erosion time constant is probably wavenumber dependent and falls in the range 200 400ma for the topographic wavelengths under consideration

effective elastic thickness variations along the andean margin and their relationship to subduction geometry 1 we present a new map of the spatial variations in effective elastic thickness te along the andes estimated using bouguer coherence the te variations reflect interactions between subducting slab and preexisting terrane structure in the forearc conductive cooling of the continent by the subducting slab exerts primary control on rigidity resulting in te that is highest 40 km where the oceanic lithosphere is oldest and coldest 20 s in the central andes te is relatively low 20 km along the volcanic chain and the altiplano and puna plateaus we interpret this weakening to reflect a high geothermal gradient maintained by advective magmatic processes a shallow and hot asthenosphere and a very weak lower crust throughout this region east of the plateaus high te delineates underthrusting of the brazilian shield finally north and south of the plateaus flat subduction areas are characterized by high te high shear wave velocity thick thermal lithosphere and low heat flow indicating that continental lithosphere there is thicker colder and stronger on the basis of these relationships we suggest that variations in slab dip along the margin relate to variations in structure of the continental lithosphere in particular we propose that upper plate structure influences the width and viscosity of the asthenospheric wedge which control the suction moment responsible for the subduction angle at depths 70 100 km for example when oceanic lithosphere subducts beneath thin continental lithosphere the low viscosity asthenosphere allows the slab to detach from the continent and sink into the mantle at normal angles however when oceanic lithosphere subducts close or beneath thick and strong continental lithosphere the asthenospheric wedge narrows and corner flow drags high viscosity mantle from the base of the thick 150 km cold continent into the wedge suction forces increase with both narrowing of the wedge and its increasing viscosity we estimate the asthenospheric viscosity underneath thick cold continents to be 1020 pa s sufficient to induce flat subduction later after prolonged hydration and weakening of the continent s base asthenospheric flow into the wedge may resume allowing the slab to sink again into the mantle at normal angles

lithosphere structure and upper mantle characteristics below the bay of bengal the oceanic lithosphere in the bay of bengal bob formed 80 120 ma following the breakup of eastern gondwanaland since its formation it has been affected by the emplacement of two long n s trending linear aseismic ridges 85oe and ninetyeast and by the loading of ca 20 km of sediments of the bengal fan here we present the results of a combined spatial and spectral domain analysis of residual geoid bathymetry and gravity data constrained by seismic reflection and refraction data self consistent geoid and gravity modeling defined by temperature dependent mantle densities along a n s transect in the bob region revealed that the depth to the lithosphere asthenosphere boundary lab deepens steeply from 77 km in the south to 127 km in north with the greater thickness being anomalously thick compared to the lithosphere of similar age beneath the pacific ocean the geoid topography ratio gtr analysis of the 85 e and ninetyeast ridges indicate that they are compensated at shallow depths effective elastic thickness te estimates obtained through admittance coherence analysis as well as the flexural modeling along these ridges led to the conclusions i 85 e ridge was emplaced in off ridge environment te 10 15 km ii the higher te values of 25 km over the afanasy nikitin seamount ans reflect the secondary emplacement of the seamount peaks in off ridge environment iii that the emplacement of the ninetyeast ridge north of 2 n occurred in an off ridge environment as indicated by higher te values 25 30 km furthermore the admittance analysis of geoid and bathymetry revealed that the admittance signatures at wavelengths 800 km are compensated by processes related to upper mantle convection

the long term strength of europe and its implications for plate forming processes field based geological studies show that continental deformation preferentially occurs in young tectonic provinces rather than in old cratons this partitioning of deformation suggests that the cratons are stronger than surrounding younger phanerozoic provinces however although archaean and phanerozoic lithosphere differ in their thickness and composition their relative strength is a matter of much debate one proxy of strength is the effective elastic thickness of the lithosphere te unfortunately spatial variations in te are not well understood as different methods yield different results the differences are most apparent in cratons where the bouguer coherence method yields large te values 60 km whereas the free air admittance method yields low values or 1 5 gyr old is much larger mean te 60 km than that of younger lithosphere mean te 30 km we propose that this strength difference reflects changes in lithospheric plate structure thickness geothermal gradient and composition that result from mantle temperature and volatile content decrease through earth s history

global and local isostatic coherence from the wavelet transform 1 a method to compute the variations in lithospheric elastic thickness te has been developed using the wavelet transform the technique which uses a superposition of two dimensional morlet wavelets in a geometry named a fan wavelet is designed to yield isotropic yet complex wavelet coefficients for the co and cross spectra of gravity and topography data these are then used to compute a spatially varying isostatic coherence from which both global and local estimates may be obtained we applied the method to synthetic gravity and topography generated for a thin elastic plate of uniform thickness 20 km yielding an apparent spatially variable te of 24 5 3 5 km the estimated global coherence for this model appears to fit the theoretical prediction as well as fourier transform based estimates and is smoother than these we also computed the wavelet coherence and hence spatially varying te for a plate of non uniform thickness yielding a difference with the model of 2 0 1 7 km

estimating the effective elastic thickness of the lithosphere of the iberian peninsula based on multitaper spectral analysis effective elastic thickness of the lithosphere of the iberian peninsula was estimated from a coherence analysis of topography and gravity spectra for which a multitaper technique was used coherence estimates provide an average te value of 17 4 km to verify the te derived from the spectral analysis we compare it to independent te estimates from strength envelopes constructed for some tectonic units located at the interior of the iberian peninsula we obtain te values ranging from 10 to 25 km these values are consistent with coherence results tectonic evolution of the area involved an early mesozoic rifting late permian early cretaceous and a subsequent period of compression present morphotectonic units result from alpine deformation events mainly neogene no widespread volcanism activity or and high heat flow values have been reported to have occurred during intraplate domain deformation the iberian lithosphere response to loading primarily reflects its pre existing inherited mechanical structure which has remained thermally relatively unperturbed

wavelet and multitaper coherence methods for assessing the elastic thickness of the irish atlantic margin summary there have been some inconsistencies in estimates of the effective elastic thickness of the continental lithosphere te based upon admittance or coherence relationships between gravity and topography this paper compares multitaper and wavelet methods to analyse the coherence between bouguer gravity and bathymetric data over the irish atlantic margin the analyses show that similar lateral te variations can be recovered from the data but demonstrate that the size of the data window can give rise to a significant downward bias in te estimates a seismically constrained 3 d gravity inversion over the rockall basin shows the presence of surface and subsurface loads whose ratio is loosely correlated with load ratio variations generated from the wavelet coherence method the te and load ratio f variations can be plausibly related to major geological structures on the margin if the load ratio variations can be interpreted geologically it implies that spectral based methods to estimate effective elastic thickness must incorporate subsurface loads within the underlying theoretical model on the irish atlantic margin te is generally low 6 18 km and is associated with a ne sw caledonian trend the weakest lithosphere is in the southern rockall basin porcupine bank and porcupine basin and the strongest lithosphere is along the rockall hatton region the low te values are consistent with results from other passive margins the reasons for such low te values on the irish atlantic margin remain unclear but may be the consequence of te being frozen into the lithosphere when loads were emplaced during continental breakup and temperature gradients were high the process of sedimentation and the presence of fluids may be contributory factors there is an indication of a geological and rheological divide between the rockall hatton region and the rockall basin possibly associated with the caledonian orogenic front

national geophysical data center candidate for the world digital magnetic anomaly map marine and airborne magnetic anomaly data have been collected for more than half a century providing global coverage of the earth furthermore the german champ satellite is providing increasingly accurate information on large scale magnetic anomalies the world digital magnetic anomaly map project is an international effort to integrate all available near surface and satellite magnetic anomaly data into a global map database teams of researchers were invited to produce candidate maps using a common pool of data sets here we present the national geophysical data center ngdc candidate to produce a homogeneous map the near surface data were first line leveled and then merged by least squares collocation long wavelengths were found to agree surprisingly well with independent satellite information this validates our final processing step of merging the short wavelength part of the near surface data with long wavelength satellite magnetic anomalies

the use of gravity anomaly data for offshore continental margin demarcation analysis of gravity anomaly data can be an effective and economical means of demarcating the world s offshore continental margins

ancient heat flow crustal thickness and lithospheric mantle rheology in the amenthes region mars surface heat flow calculations for the amenthes region of mars can be independently performed using the depth to the brittle ductile transition and the effective elastic thickness of the lithosphere estimated for the late noachian early hesperian equivalent to an estimated absolute age of 3 6 3 8 ga this along with crustal heat production rates estimated from heat producing elements abundances permits us to put constraints for that particular place and time on both the thermal and mechanical properties of the lithosphere and the crustal thickness the depth to the brittle ductile transition deduced from modeling of the topography of amenthes rupes is 27 35 km and the associated surface heat flow is 26 37 mwm 2 on the other hand the effective elastic thickness in this region is between 19 and 35 km the surface heat flow deduced by considering crustal and lithospheric mantle contributions to the total lithospheric strength as well as wet or dry olivine for lithospheric mantle rheology is 31 49mwm 2 the relatively limited overlap among te andzbdt based heat flowvalues implies a surface heat flowof 31 36mwm 2 with a high fraction originated from crustal heat sources and awet mantle rheology the so obtained local crustal thickness is 43 74 km which suggests an average thickness of 40 75 km for the martian crust for the frequently used crustal density of 2900 kgm 3 our results suggest a crustal thickness of 50 63km for theamenthes region and an average crustal thickness of 45 65 km for mars