the relationship between depth age and gravity in the oceans we reassess the applicability of the thermal plate cooling model to the subsidence of the north pacific atlantic and north indian ocean basins we use a new numerical plate model in which the thermophysical parameters of the lithosphere vary with temperature according to the results of laboratory experiments and the ridge temperature structure is consistent with the thickness of the oceanic crust we first attempt to exclude thickened crust from our data set and then to exclude swells and downwellings by masking regions of the data that remains that have significant gravity anomalies when there exists a clear regional correlation between intermediate wavelength gravity and topography we find that the average variation of depth with age is consistent with conventional half space models until about 90 myr thereafter the departure from the half space cooling curve is more rapid than predicted using simple conductive plate cooling models the depth age curves in the pacific and atlantic show 250 m of temporary shallowing between the ages of 90 130 myr a result consistent with the outcome of experiments on the initiation of small scale boundary layer convection the results do not change significantly if the estimated component of the gravity arising from plate cooling is subtracted prior to calculation of the correlation between gravity and topography a 90 km thick conductive plate is nevertheless a reasonable model for the average temperature structure of the oldest part of the pacific ocean lithosphere in the pacific the broad topographic undulations associated with the line island swell the hawaiian swell and surrounding basins have correlated gravity anomalies and an admittance of approximately 30 mgal km 1 and are likely to result from convective circulation in the upper mantle in the northeast atlantic the intermediate wavelength admittance over the cape verde swell is similar in the northwest atlantic over the bermuda swell it is slightly larger but not as well constrained

estimates of te for continental regions using goce gravity satellite only gravity fields and surface gravity obtained from altimetric measurements now agree well at wavelengths greater than 180 km satellite gravity fields can therefore be used to estimate the elastic thickness tete in regions where surface observations are sparse they are used for this purpose in a number of continental regions of india africa and antarctica where the topography is sufficiently rough and also in regions of the usa china australia and siberia where there are surface measurements estimates of tete for antarctica depend on measurements of ice thickness which are now available for much of the continent values of tete are obtained using two methods from the admittance between the free air gravity and the topography and from the coherence between bouguer gravity anomalies and the topography the first but not the second gives values of tete that are everywhere less than the seismogenic thickness where there is sufficient topography estimates of tete from precambrian shields are all greater than 10 km and do not correlate with the lithospheric thickness they are probably are governed by variations in crustal heat generation rates values for regions strongly affected by phanerozoic tectonics are all less than 7 km and all such regions are underlain by thin 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

short wavelength high amplitude gravity anomalies around the banda sea and the collapse of the sulawesi orogen abstract in eastern indonesia high density ophiolitic rocks outcropping on islands surrounding the banda sea are in many cases associated with strong gravity anomalies and steep gravity gradients however the relationships are not always straightforward bouguer gravity levels and gradients over the extensive east sulawesi ophiolite are generally relatively low although short wavelength high amplitude anomalies indicate rapid changes in thickness of high density rocks in a few places in the banda arc most local positive anomalies due to ophiolites are superimposed on a steep regional gravity gradient but in one case in western seram there is a distinct and important spatial separation between the two on buru west of seram a gradient of more than 10 mgal km testifies to the presence of very dense rocks in the near subsurface despite the absence of ophiolites in the outcrop gravity variations and ophiolite distribution around the banda sea are compatible with extension having occurred in the sulawesi region following and as a result of oligo miocene collision with an australian derived microcontinent similar histories have been proposed for many mediterranean deep basins of similar size shape and character and emplacement of some of the high density masses in the banda arc has probably resembled at least the later stages in the emplacement of peridotite massifs in the rif betic belt in both areas the present close association of the ultramafic rocks and their associated local anomalies with a strong regional gravity gradient is largely coincidental

spatial variations in te in the southern appalachians eastern united states various studies in the oceans have shown that the flexural rigidity or equivalently effective elastic thickness te of the lithosphere is determined by the load and plate age the results of studies in the continents have however been more controversial determinations of te made using spectral studies and based on the bouguer anomaly coherence technique and surface and subsurface loading suggest that north america has a high te core 100 km which is flanked by lower values in contrast studies based on the free air admittance suggest that te of north america is 25 km and of the order of the seismogenic layer thickness it has been proposed that this discrepancy results from the fact that estimates based only on bouguer coherence may be biased upward due to topographical erosion which introduces noise especially at short wavelengths in order to address this question further we have used a maximum entropy based coherence method to determine the wavelength relationship between gravity and topography in the southern appalachians a region where the bouguer coherence free air admittance and forward modeling techniques have already been applied our studies reveal a variable te structure with a mean te of 51 km and values which have a range 20 to 100 km the mapped te fabric has a distinct ne sw trend which appears to follow the tectonic elements of the southern appalachians in particular the foreland is generally associated with higher te values than the flanking orogenic belt correlations at smaller scales are difficult however to establish the te fabric does not reflect the complex terrains that make up highly deformed regions within the orogenic belt our spectrally determined te estimates are in close agreement with ones based on forward modeling they are a factor of 4 higher however than results previously based on free air admittance we attribute this to the fact that we have used the bouguer coherence technique which accounts for both surface and subsurface loading our results suggest that while other factors such as erosion may upwardly bias te in some regions buried loads satisfactorily explain the spectral estimates in the southern appalachians irrespective of whether they are based on bouguer coherence or free air admittance there is solid geological evidence in the southern appalachians which is readily attributable to the presence of subsurface loads this is a strong argument we believe to suggest that te in the continents is high and can have values which range from 20 to 100 km

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

volcanism and tectonics on venus we review recent developments in the study of volcanism and tectonics on venus venus s crust is basaltic dry and probably about 30 km thick the mantle convects giving rise to plumes and has a similar composition and mean temperature 1300 c but a higher viscosity 10 20 pa s than that of the earth inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km the elastic thickness of the lithosphere is about 30 km on average the present day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle most of the differences between earth and venus processes can be explained by the absence of water venus underwent a global resurfacing event 300 600 ma ago the cause and nature of which remains uncertain the present day surface heat flux on venus is about half the likely radiogenic heat generation rate which suggests that venus has been heating up since the resurfacing event

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 estimates for venus using line of sight accelerations from magellan cycle 5 abstract the elastic thickness te for various regions of venus is estimated by comparing the observed line of sight los acceleration of the magellan spacecraft with that predicted using a spherical harmonic representation of the topography to degree and order 360 at long wavelengths typically longer than about 500 km the transfer function between the topography and gravity or admittance usually has a flat spectrum with a magnitude of between 20 and 50 mgal km 1 which is most likely due to convective support in particular the topographic highs associated with beta phoebe bell and eistla are thought to be dynamically supported at shorter wavelengths the admittance increases suggesting a component of flexural support the elastic thicknesses are constrained by fitting theoretical admittance curves to the observed short wavelength values for the admittance results from magellan cycle 5 show evidence of regional variations in elastic thickness between about 19 and 29 km with a mean value of around 21 23 km assuming a crustal thickness of 16 km and a density of 2670 kg m 3 the observed variations in admittance between different regions are unlikely to be due to differences in crustal thickness or density and probably represent real variations in te the values obtained are similar to those from an identical analysis using cycle 4 data estimates of the elastic thickness of the ovda and alpha regions are unreliable probably because the topography is not well determined no reliable estimates of elastic thickness could be made from cycle 6 data where the altitude of the spacecraft was higher than about 300 km due to the reduction in short wavelength signal amplitude with altitude

the effective elastic thickness of the india plate from receiver function imaging gravity anomalies and thermomechanical modelling summary the range and the meaning of the effective elastic thickness eet in continental areas have been subject to controversy over the last two decades here we take advantage of the new data set from the hi climb seismological experiment to re estimate the eet of the india plate along a south north profile extending from the ganges basin to central tibet receiver functions give a high resolution image of the base of the foreland basin at 5 km depth and constrain the crustal thickness which increases northwards from 35 km beneath the indo gangetic plain to 70 km in southern tibet together with available data sets including seismic profiles seismological images from both indepth and himnt experiments deep well measurements and bouguer anomaly profiles we interpret this new image with 2 d thermomechanical modelling solutions using different type of crustal and mantle rheologies we find that 1 the eet of the india plate decreases northwards from 60 80 to 20 30 km as it is flexed down beneath himalaya and tibet due to thermal and flexural weakening 2 the only resistant layer of the india plate beneath southern tibet is the upper mantle which serves as a support for the topographic load and 3 the most abrupt drop in the eet located around 200 km south of the mft is associated with a gradual decoupling between the crust and the mantle we show that our geometrical constraints do not allow to determine if the upper and lower crust are coupled or not our results clearly reveal that a rheology with a weak mantle is unable to explain the geometry of the lithosphere in this region and they are in favour of a rheology in which the mantle is strong

an estimation of the elastic thickness of the lithosphere in the canary archipelago using admittance function a recent compilation of shipborne gravity and bathymetry data has been used to determine the effective elastic thickness eet of the lithosphere beneath the canary islands volcanic complex by means of the admittance technique as a first approach we have computed the admittance which is interpreted in terms of isotropic elastic plate thickness with an eet of 23 km nevertheless the proximity of the west african continental margin wacm together with the lateral variability in lithosphere structure in part caused by the large volcanic history of the canaries require a slightly different approach to minimize the influence of the margin and to check whether or not any significant variation in eet occurs related to the known canary hotspot we have computed the admittance following the isochron lines the results show a higher eet 35 km than that obtained by the first approach but slightly smaller than that predicted by the cooling plate model

seismicity structure and rheology of the lithosphere in the lake baikal region this paper combines observations of seismicity gravity topography and thermal and velocity structures to investigate the rheological properties of the lithosphere in the lake baikal region we examine the seismogenic thickness ts using 25 earthquakes of mw 5 1 7 1 whose full source parameters have been determined by inversion of teleseismic waveforms 13 of which are presented here for the first time these 25 events plus six others mw 5 0 5 8 whose depths are well constrained show that moderate earthquakes occur at depths up to 30 km in the northeast baikal rift based on the teleseismic waveform modelling results and published relocations of microearthquakes using regional networks we conclude that the mantle is not a significant source of seismicity in the baikal region using the admittance between free air gravity and topography we estimate the effective elastic thickness te in the region to be between 5 and 20 km nowhere do the data require that te ts consistent with the simple interpretation that the long term strength of the lithosphere resides in its seismogenic layer a weak mantle in the baikal region can be explained by its high temperature which we estimate by combining local geotherm estimates with the regional upper mantle velocity structure obtained from fundamental and higher mode surface waves geotherms are fitted to pressure and temperature estimates from mantle nodules at four sites both within and outside the siberian shield in order to constrain the temperatures at the moho we estimated crustal thicknesses using teleseismic receiver functions moho temperatures are estimated to exceed 550 c beneath the siberian shield and are higher in the more recently deformed mountain belts to the south based on a reassessment of oceanic geotherms and seismicity it seems likely therefore that the mantle in the baikal region is too hot to be a source of long term strength this is consistent with the recent suggestion that the distribution of mantle seismicity in both the oceans and the continents is dependent on temperature alone finally we note that results from s wave tomography studies combined with the observed locations of rift related earthquakes lead us to suspect that the frequently published position of the edge to the siberian shield at the surface provides a poor description of that same boundary at depth

reappraisal of the effective elastic thickness for the sub andes using 3 d finite element flexural modelling gravity and geological constraints summary estimates of effective elastic thickness te for the western portion of the south american plate using independently forward flexural modelling and coherence analysis suggest different thermomechanical properties for the same continental lithosphere we present a review of these te estimates and carry out a critical reappraisal using a common methodology of 3 d finite element method to solve a differential equation for the bending of a thin elastic plate the finite element flexural model incorporates lateral variations of te and the andes topography as the load three te maps for the entire andes were analysed stewart watts 1997 tassara et al 2007 and perez gussinyeet al 2007 the predicted flexural deformation obtained for each te map was compared with the depth to the base of the foreland basin sequence likewise the gravity effect of flexurally induced crust mantle deformation was compared with the observed bouguer gravity te estimates using forward flexural modelling by stewart watts 1997 better predict the geological and gravity data for most of the andean system particularly in the central andes where te ranges from greater than 70 km in the sub andes to less than 15 km under the andes cordillera the misfit between the calculated and observed foreland basin subsidence and the gravity anomaly for the maranon basin in peru and the bermejo basin in argentina regardless of the assumed te map may be due to a dynamic topography component associated with the shallow subduction of the nazca plate beneath the andes at these latitudes

toward mapping the effective elastic thickness of planetary lithospheres from a spherical wavelet analysis of gravity and topography abstract the effective elastic thickness t e of the lithosphere controls the flexural response to transverse loading and can be used in conjunction with rheological models to remotely estimate surface heat flux of terrestrial planets in the vast majority of studies t e estimation is carried out in a two step process 1 the joint spectra admittance and or coherency of gravity anomalies free air and or bouguer and topography are calculated within finite size windows and 2 the spectra are inverted using a model for the loading of an infinite elastic plate or shell in recent years research in the spatio spectral analysis of cartesian grids and improvements in lithospheric loading models have allowed the mapping of t e on the earth at unprecedented resolution nevertheless the limitations imposed by working with cartesian data and models are hampering further advances in terrestrial t e studies the planetary community on the other hand has traditionally used spatio spectral methods that work directly on the sphere thereby avoiding the undesirable distortions and biases inherent in cartesian studies however the models and methods developed for the earth have never been applied to planetary data therefore also limiting further advances in the mapping of planetary t e this paper reviews the latest developments in t e studies and proposes to combine advances in both terrestrial and planetary studies to allow the mapping of t e of terrestrial planets using a spherical wavelet analysis of gravity and topography data we begin with a review of the recent literature on the challenges and debates concerning t e estimation from joint gravity and topography spectra we then briefly review the implementation of the continuous planar and spherical wavelet transforms cpwt and cswt and demonstrate that both transforms possess exactly the same resolving power however the cswt does not suffer from effects associated with non periodic boundaries we further re derive in a consistent manner the expressions for the admittance and coherency from flexural equations for the loading of thin elastic plates and shells and show that they result in similar spectra except at the longest wavelengths where the shell membrane stresses dominate we then estimate global grids of earth s continental t e by inverting the planar and spherical admittance and coherency functions both separately and jointly using either free air or bouguer gravity anomalies the correspondence between the cartesian and spherical approaches and the similarity of t e results for the earth s continents indicate that the spherical methods are robust results obtained from the joint inversion of admittance and coherency show that simple lithospheric loading models fail to capture the complexity of the data with adverse effects on the estimated parameters finally we extend the analysis to the moon and other terrestrial planets and discuss limitations and future applications of the fully spherical techniques

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

dolerites of the woodlark basin papuan peninsula new guinea a geochemical record of the influence of a neighbouring subduction zone abstract the moresby seamount located in the woodlark basin is a fragment of oceanic crust dolerites and gabbros generated 66 4 1 5 my ago before being obducted during the eocene on the australian margin since 8 my the motion of a normal fault related to the opening of the woodlark basin is responsible for the unroofing of the moresby seamount the latter was sampled during the odp campaign leg 180 in 1998 geochemical compositions of dolerites can be explained by a process of fractional crystallization in which the magnesian spinel was involved therefore fractionation processes took place at relatively high pressure 1 gpa most likely during the upward migration of deep magmas throughout the mantle the fe rich gabbros more differentiated than the dolerites could have derived from an early differentiated magma evolving in the crust at relatively low pressures the trace element content of dolerites indicates that their source was a depleted oceanic mantle slightly influenced by arc related magmas this study suggests that the ophiolites fragment of the moresby seamount was created in the vicinity of a subduction zone this model being in agreement with the regional tectonic setting of se asian since the cainozoic

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

back arc basin origin for the east sulawesi ophiolite eastern indonesia the east sulawesi ophiolite is one of the three largest ophiolites in the world it displays all the components of a typical sequence from residual mantle peridotites to cumulate gabbros sheeted dolerites and lavas of normal mid oceanic ridge basalt morb composition trace element data on the lavas and dolerites and particularly their depletion in nb compared to neighboring incompatible elements suggest a subduction zone environment for their origin the chemical similarity between the east sulawesi ophiolite lavas and those from the eocene celebes sea back arc basin crust together with their identical age strongly suggest a back arc tectonic environment for this ophiolite which represents a fragment of the eurasian plate obducted onto the east sulawesi basement of australian origin

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

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

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

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

a reexamination of the flexural deformation beneath the hawaiian islands seismically derived depth estimates to the top of the oceanic crust beneath the hawaiian islands indicate that the curvature of the deflected lithosphere is much larger than commonly believed the conservative and model independent curvature estimates exceed 10 7 m 1 and are comparable in magnitude to curvatures at trenches and outer rise systems the depth estimates are used to constrain both two dimensional 2 d and three dimensional 3 d flexural models the curvature constraints require a 2 d variable elastic thickness that decreases from 35 km in areas away from the volcanic load to 25 km directly beneath the load in an attempt to understand the nature of the yielding beneath the hawaiian islands we introduce two new 3 d models the first model combines a realistic yield strength based rheology with a new technique for 3 d flexure calculations in which the elastic plate thickness is curvature dependent the new variable rigidity model predicts an undeformed mechanical plate thickness of 44 km decreasing to 33 km beneath the big island of hawaii the best fitting mechanical thickness corresponds approximately to the depth to the 600 c isotherm in 90 m y old lithosphere the second model uses a broken plate but here the crack is oriented along the weak molokai fracture zone rather than along the island chain trend this unconventional flexure model can explain the observed asymmetry in the depth data across the fracture zone without requiring the excessively large elastic thickness of more conventional broken plate models both the proposed models imply that modeling with constant thickness plates may underestimate the true mechanical plate thickness by being unduly influenced by the weak zone beneath the seamounts

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

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

flexure of venusian lithosphere measured from residual topography and gravity received 19 september 2000 revised 3 august 2001 accepted 20 september 2001 published 26 february 2002 1 the elastic lithosphere thickness te for various locations on venus is estimated by modeling lithospheric flexure associated with rifts coronae chasmata and the moats visible around certain large volcanoes by modeling flexure using the residual topography a range of elastic thicknesses was found from around 10 to 40 km or greater a number of these values are not well determined and only constrain te to be 10 km the shear stresses predicted from the topography given these values for te reach several hundred mpa with surface faulting visible in the synthetic aperture radar sar images at many locations the elastic thickness was also estimated at seven volcanolike structures by modeling the gravity predicted from the observed topography this yielded elastic thickness estimates varying between approximately 20 and 60 km which were generally more tightly constrained however an elastic thickness of 25 km fits almost all the observed profiles within uncertainty and the results from modeling the gravity yield an average global elastic thickness of 29 6 km there is no evidence that the elastic thickness falls anywhere outside this range the lack of large scale regional variations in te on venus in contrast to the situation observed on the earth is consistent with a lack of water and plate tectonics on venus index terms 6295 planetology solar system objects venus 8149 tectonophysics planetary tectonics 5475 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1227 geodesy and gravity planetary geodesy and gravity 5420 5714 6019 keywords venus elastic thickness lithosphere flexure residual topography gravity

measurements of the elastic thickness under ancient lunar terrain abstract the part of a planet s gravity that is coherent with its topography provides information about the deflection of its crust after loading and hence the resistance of its lithosphere to bending at the time the load was emplaced we used observed line of sight accelerations from lunar prospector together with the accelerations we would have expected if anomalies in the gravity field were only caused by topography to estimate the admittance and coherence between topography and gravity over several regions of the lunar nearside we then compared our estimates to what we would expect if the lithosphere behaved as a floating elastic plate or shell assuming a linear relationship between topography and gravity we found in the region surrounding the southern highland crater clavius that the data can be modeled using a thin plate with t e 12 5 km and uncorrelated loads at the surface and base of the upper crust a spherical shell model with surface loading is less satisfactory to fit the admittance adequately requires topography with wavelengths over 400 km to be formed when t e 1 km and the remainder when t e 7 5 km by contrast the apparent lack of compensation around the youngest giant impact basins requires a plate with t e 80 km or a shell with t e 25 km our results indicate the thickness of the lunar lithosphere increased from 12 km in the pre nectarian to 25 km in the nectarian

mesozoic porphyry cu au mineralization and associated adakite like magmatism in the philippines insights from the giant atlas deposit the atlas porphyry cu au deposit is located in central cebu island in the philippines with a proven mineral reserve of 1420 mt at 0 45 cu 0 24 g t au 0 018 g t mo and 1 8 g t ag it is associated with the lutopan quartz diorite porphyry stock that was emplaced into the cretaceous volcano sedimentary sequences of the cansi formation two stages of hypogene alteration have been identified at atlas stage i quartz magnetite chalcopyrite pyrite veins associated with potassic and propylitic alteration and stage ii anhydrite pyrite chalcopyrite specularite veins associated with phyllic alteration supergene gypsum veins are also present mineralization is mainly associated with the stage i and stage ii veins the lutopan quartz diorite porphyry yielded a zircon u pb age of 108 5 1 6 ma and is characterized by high sr y 54 69 with the corresponding magma having high oxygen fugacity avg fmq 2 7 avg ce4 ce3 of zircon 439 and h2o concentrations presence of amphibole and biotite phenocrysts and low magmatic temperatures avg ti in zircon thermometry 706 c the relatively low la yb ratios 9 11 listric shaped normalized ree patterns with weak or absent eu anomalies together with zircon ehf t values of 3 4 to 10 0 whole rock isr values of 0 70373 0 70382 end t values of 2 4 3 4 and mg numbers of 49 53 suggest that the lutopan quartz diorite porphyry was likely derived from partial melting of young lower continental crust or interaction between asthenospheric melts and lower crust melts that evolved to adakite like trace element compositions through amphibole fractionation stage i veins yielded a pyrite re os isochron age of 110 2 5 6 ma making atlas the only mesozoic porphyry deposit discovered in the philippines

on the interpretation of frequency response functions for oceanic gravity and bathymetry summary oceanic bathymetry and gravity comprise a geophysical system which is often characterized by a linear frequency response function or admittance the actual behaviour of this system differs in important respects however from that of an ideal linear model three sources of such differences are considered in this study the presence of noise in the data non linearity due to finite amplitude and intrinsic two dimensionality of the data fields the high positive correlation between admittance amplitude and coherence estimates obtained in a number of studies indicates that the noise is not normally distributed with zero mean as generally assumed non linear modification of the admittance results from the finite amplitude of the bathymetry and the non linear elasticity of the lithosphere the former effect increases with bathymetric roughness and is most pronounced for shortwavelength uncompensated bathymetry the latter is significant only if the deflection of the lithosphere is comparable to its elastic thickness as for certain large seamounts in the pacific ocean which were emplaced on lithosphere near ridge crests the most severe problem arises for features which are markedly two dimensional features typical of observed oceanic seamounts are characterized by admittances along tracks which differ substantially from those appropriate to one dimensional bathymetry this difference occurs primarily in the long wavelength portion of the admittance rendering problematic the comparison of observed admittances or gravity anomalies with theoretical expressions

the elastic thickness of the british isles free air gravity and topography data from the british isles have been analysed in the frequency domain to determine the mechanisms of topographic support first we calculate how the admittance between measured free air gravity and topography varies as a function of wavelength a simple model which consists of a crust with two layers of different densities overlying a higher density mantle was then used to calculate admittance functions the misfit between observed and calculated admittance was minimized by varying elastic thickness and the proportion of the total load due to internal loading at short wavelengths c 0 2 the general behaviour of the observed admittance suggests that topography is primarily supported by elastic stresses in the crust a small departure between observed and calculated admittance at the longest wavelengths 400

the decay of the spectrum of the gravitational potential and the topography for the earth summary the spectrum of the earth s gravitational potential and topography as represented by spherical harmonic expansions to degree 180 have been computed modelling the decay in the form of al values of a and for several degree l ranges were computed for degree range 5 180 was 2 54 for the potential and 2 16 for equivalent rock topography the potential decay was somewhat slower than that i e 3 implied by kaula s rule however at high degree ranges the values were larger 3 20 for degrees 101 180 agreeing better with recent determinations from terrestrial gravity data and geoid undulations implied by satellite altimetric data the values imply that the potential decays faster at higher l values the values of for topography were fairly uniform around 2 which agrees with a suggestion made by vening meinesz in 1951 we also found that the value for the earth s potential agrees well with the value implied by the topography with airy isostatic compensation with the depth of compensation equal to 30 km however the magnitude of the power implied by the topographic isostatic potential was approximately one third of the observed potential

lithosphere rheology and active tectonics in mongolia relations between earthquake source parameters gravity and gps measurements in this paper we use observations of earthquake source parameters and gravity to investigate the mechanical properties and the active faulting of the lithosphere in mongolia well determined earthquake centroid depths including 10 from inversions of p and sh waveforms that are presented here for the first time show that the seismogenic thickness ts within mongolia itself is less than 20 km however to both the east in parts of the lake baikal rift system and the west adjacent to the junggar basin and kazakhstan platform the seismogenic thickness is considerably greater and includes essentially the whole crust from the admittance between the free air gravity and the topography and also from profiles across a flexural foreland basin we determine the effective elastic thickness te in central mongolia to be 30 km in both mongolia and its surroundings te is comparable with ts and where it is well determined te ts these data are consistent with the view that the strength of the continental lithosphere resides in its seismogenic part which in mongolia is the upper crust but to both the east and west appears to be the whole crust the earthquake source parameters also allow us to ask how the active faulting in mongolia accommodates the velocity field revealed by gps measurements it is likely that the entire mongolian altay range in the west rotates counter clockwise relative to stable asia and is responsible for the distributed e w left lateral shear seen further east in central mongolia the admittance observations show no evidence at the present day for convective mantle support or a hotspot responsible for the elevated region of the hangay dome in central mongolia even though the geochemical data from nodules in late cenozoic basalts and seismic tomography studies suggest elevated temperatures at shallow depths 125 km and probably thinned lithosphere

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

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

effective elastic thickness along the conjugate passive margins of india madagascar and antarctica a re evaluation using the hermite multitaper bouguer coherence application abstract gondwana correlation studies had rationally positioned the western continental margin of india wcmi against the eastern continental margin of madagascar ecmm and the eastern continental margin of india ecmi against the eastern antarctica continental margin eacm this contribution computes the effective elastic thickness te of the lithospheres of these once conjugated continental margins using the multitaper bouguer coherence method the results reveal significantly low strength values te 2 km in the central segment of the wcmi that correlate with consistently low te values 2 3 km obtained throughout the entire marginal length of the ecmm this result is consistent with the previous te estimates of these margins and confirms the idea that the low te segments in the central part of the wcmi and along the ecmm represents paleo rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the marion hotspot and lithospheric extension during the rifting the uniformly low te value 2 km along the eacm indicates a mechanically weak lithospheric margin probably due to considerable stretching of the lithosphere considering the fact that this margin remained almost stationary throughout its rift history in contrast the ecmi has comparatively high te variations 5 11 km that lack any correlation with the regional tectonic setting using gravity forward and inversion applications we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere the study concludes that the thick pile of the bengal fan sediments in the ecmi masks and has erased the signal of the original load induced topography and its gravity effect has biased the long wavelength part of the observed gravity signal the hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling which likely accounts a relatively high te estimate

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

the relationship between martian gravity and topography the relationship between gravity and topography of various regions of mars is used to estimate their effective elastic thicknesses te using direct measurements of line of sight velocity rather than spherical harmonic coefficients estimates of te vary from 70 km for tharsis 29 km for elysium to 14 5 km for the southern hemisphere and show that the thickness of the martian lithosphere increases with age as the radioactive isotopes of k th and u decay a simple parameterised model of the convective thermal history is used to estimate the temperature structure of the lithosphere and shows that the base of the elastic layer has a temperature of 300 50 c or similar to the value for terrestrial continents in both cases the rheology is probably affected by the presence of water the short wavelength behaviour of the gravity field allows the density of the rocks that form the topography to be estimated and gives values of about 3 0 mg m3 for tharsis and elysium this value is substantially greater than that of 2 7 mg m3 obtained for earth and is in agreement with estimates from snc shergottites nahklites chassigny meteorites of 3 3 mg m3 the density of the topography of valles marineris is only 2 35 mg m3 and suggests that ice may be present below the surface in the heavily bombarded southern hemisphere isostatic compensation occurs at wavelengths as short as 700 km which requires the effective compensation depth to be no more than 10 km the gravity field with wavelengths greater than 1500 km may be supported dynamically by a plume rising beneath the tharsis region the difference in temperature between the solidus and the present areotherm is less than 250 c so melt generation can occur in rising plumes

a seismic reflection profile study of lithospheric flexure in the vicinity of the cape verde islands 1 seismic reflection profile data are used to determine the stratigraphic architecture of the flexural moat that flanks the cape verde islands the moat region is characterized by upward of 1 2 km of poorly to well stratified material the two lowermost units thicken from west to east and are attributed to sediment loading and flexure at the nearby west africa continental margin during early cretaceous to early miocene the two uppermost units in contrast thicken concentrically around the islands and are attributed to the infilling of a flexural moat that formed by volcanic loading since the early miocene flexure modeling shows that the thickness of the moat infill cannot be explained only by surface loading and requires that the downward flexure due to surface loads is opposed by an upward acting subsurface load the best fit combined surface and buried loading model is for an elastic thickness te of 29 km a load and infill density of 2700 kg m 3 and a ratio of surface to subsurface loads f of 0 2 these results are in accord with spectral studies of free air gravity anomaly and topography data based on the admittance technique the subsurface loads are spatially limited to the islands and their submarine flanks nevertheless they are associated with a broad regional uplift of up to 400 m the uplift is large enough to explain why the moat infill is generally tilted away from rather than toward the islands the uplift is too small however to account for the height of the swell upon which the cape verde islands are superimposed the origin of the cape verde swell is not known however a normal te and a modest heat flow anomaly suggest that the swell cannot be fully explained by uplift due to thermal reheating of the lithosphere by an underlying hot spot and that other deep seated mantle processes must be involved

estimates of te from goce data abstract the cold upper part of the lithosphere can support the elastic stresses that govern rigid plate movements and support the earth s topography in many regions the thickness t e of the layer involved can be estimated from the relationship between surface gravity measurements and topography however there are extensive regions of the continents where the topography is well determined but the gravity field is not the obvious solution to this problem is to use the satellite rather than the surface gravity field the wavelength range of interest 200 500 km requires a satellite with a low orbit the first satellite to satisfy this requirement is the gravity field and steady state ocean explorer goce measurements of the elements of the gravity gradient tensor and determinations of the spherical harmonic coefficients of the gravity field based on goce observations are used to estimate t e for hawaii these estimates are 18 4 km and 21 4 km respectively and agree well with a value of 18 2 km obtained from the surface gravity field determined from altimetry an attempt to estimate the value of t e of the central north atlantic using goce data was not successful whereas surface data gives a value of about 4 km estimates for tibet and surrounding regions where the gravity field from surface measurements is poorly determined are 25 32 km the nw part of s america gives 17 24 km the difference in the values of t e from the two regions probably results from the difference in lithospheric thicknesses estimates of t e from central and southern africa of 31 34 km are less well constrained than are those from tibet and s america these results show that the data from goce can be used to estimate the value of t e where it is poorly determined from surface measurements is greater than 15 km and where large topographic loads produce large gravity anomalies

a new analytical solution estimating the flexural rigidity in the central andes summary we present a new 2 d analytical solution of the fourth order differential equation which describes the flexure of a thin elastic plate the new analytical solution allows the differential equation for an elastic plate to be solved for any irregular shaped topography with a high spatial resolution we apply the new method to the central andes the flexural rigidity distribution calculated by this technique correlates well with tectonic units and the location of fault zones for example the central andean gravity high correlates with the presence of a rigid high density body

spinel compositions of mantle hosted chromitite from the eastern anatolian ophiolite body turkey implications for deep and shallow magmatic processes abstract the eastern anatolian accretionary complex includes several ophiolitic megablocks and or tectonic slivers within a 150 180 km long east west trending complex formed during the late cretaceous tertiary in eastern turkey the alabayir mollatopuz yukaribalcikli and mehmetalan ophiolites are megablocks or tectonic slivers containing locally massive nodular or schlieren banded chromitite layers these podiform chromitites formed in mantle sequences and are classified as high cr chromitites cr 0 63 0 88 mg 0 50 0 64 0 01 0 5 wt tio 2 5 7 18 8 wt al 2 o 3 calculated parental melt compositions of these chromitites indicate boninite magma characteristics 8 2 13 4 wt al 2 o 3 0 64 1 50 feo mgo the pge patterns also support the view that they crystallized from a boninitic melt the total pge contents of chromitites vary between 79 and 390 ppb pd ir ratios of chromitites vary from 0 07 to 0 28 and are consistent with an ipge fractionated nature the examined chromitites were divided into two groups in terms of their mineral chemistry group i alabayir mollatopuz yukaribalcikli and mehmetalan i chromitites group ii mehmetalan ii chromitites mineral chemistry of these chromitites and their parental melt composition indicate that group i chromitites were probably formed at shallow mantle depths and that group ii chromitites formed in deeper parts of the mantle mineral chemistry of these eastern anatolia high cr chromitites indicate that they formed not only in the deeper mantle but also in shallower parts of the moho transition zone

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

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

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

high resolution local moho determination using gravity inversion a case study in sri lanka the seismic data incorporated in global moho models are sparse and therefore the interpolation of global moho depths on a local area may give unrealistic results especially in regions without adequate seismic information gravity inversion is a useful tool that can be used to determine moho depths in the mentioned regions this paper describes an interactive way of local moho depth determination using the gravity inversion method constrained with available seismic data before applying inversion algorithms the bouguer gravity data is filtered in various stages that reduce the potential bias usually expected in moho depth determination using gravity methods with constant density contrast assumption a test area with reliable seismic data is used to validate the results of moho computation and subsequently the same computation procedure is applied to the sri lankan region the results of the test area are in better agreement with seismically determined moho depths than those obtained by global moho models in the sri lankan region moho determination reveals a fairly uniform thin crust of average thickness around 20 km the overall result suggests that our gravity inversion method is robust and may be suitable for local moho determination in virgin regions especially those without sufficient seismic data

modelling plume related uplift gravity and melting on venus abstract three roughly circular regions on venus about 1000 km across are identified as potential hawaii scale plume sites on the basis of their gravity 70 90 mgal and topography 1 6 2 0 km anomalies and signs of melt generation and rifting axisymmetric isoviscous convection models are used to reproduce gravity profiles across these plumes and the line of sight acceleration of the magellan spacecraft as it passes over them the best fitting models have a conductive lid thickness of less than 150 km mantle viscosity of 8 9 1019 to 9 6 1020 pa s and a basal heat flux of 15 25 m w m 2 the lid thickness is constrained by requiring a modest amount of melt generation and a potential temperature of about 1300 c the high mantle viscosity relative to that of earth is probably a consequence of the absence of water in the mantle and may help to explain the current absence of plate tectonics on venus the low heat flux suggests that the thermal evolution of venus has differed from that of the earth

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

spatial and temporal uplift history of south america from calibrated drainage analysis a multi disciplinary approach is used to analyze the cenozoic uplift history of south america residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present day dynamic topography admittance analysis and crustal thickness measurements indicate that the elastic thickness of the borborema and altiplano regions is 10 km with evidence for sub plate support at longer wavelengths a drainage inventory of 1827 river profiles is assembled and used to investigate landscape development linear inverse modeling enables river profiles to be fitted as a function of the spatial and temporal history of regional uplift erosional parameters are calibrated using observations from the borborema plateau and tested against continent wide stratigraphic and thermochronologic constraints our results predict that two phases of regional uplift of the altiplano plateau occurred in neogene times regional uplift of the southern patagonian andes also appears to have occurred in early miocene times the consistency between observed and predicted histories for the borborema altiplano and patagonian plateaux implies that drainage networks record coherent signals that are amenable to simple modeling strategies finally the predicted pattern of incision across the amazon catchment constrains solid sedimentary flux at the foz do amazonas observed and calculated flux estimates match suggesting that erosion and deposition were triggered by regional andean uplift during miocene times

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

flexure of the continental lithosphere with multilayered rheology summary in this paper a model of flexure of the continental lithosphere is derived taking into account crustal and mantle rheologies bending of the continental lithosphere is modelled with a double yield stress envelope three layers brittle elastic and ductile for the crust and three analogous layers for the mantle portion the deformation of the layers is controlled by the rheological properties of quartz rich crustal rocks and olivine rich mantle rocks the influence of various factors such as the depth of moho strain rates thermal structure of the lithosphere boundary conditions and topographic load is examined results show that the mechanical strength of the continental lithosphere in the horizontal and vertical directions is primarily controlled by the present thermal structure of the plate boundary forces and moments and the applied topographic load this explains why mountainous regions may be more locally compensated than adjacent regions we also thus are able to explain why many continental plates have apparent effective rigidities much smaller than those predicted on the basis of their geological ages the model is then applied to the tien shan tarim area central asia and original topography and gravity data are used to constrain parameters of the model we found that the model satisfactorily matches the data and is also able to predict the thermal state of the plate and the location of the deep seismicity

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

dynamic topography and gravity anomalies for fluid layers whose viscosity varies exponentially with depth summary we have derived analytic integral relations between boundary topography and temperature as a function of wavenumber for a fluid layer whose viscosity varies exponentially with depth similar relations between gravity and temperature are also derived it is found that when the viscosity changes little over the depth of the layer less than an order of magnitude the topography kernels for both the surface and bottom boundaries are similar to those for the isoviscous case the differences being more pronounced at intermediate wavelengths twice the layer depth at these wavelengths the extrema of the gravity kernel can be altered by as much as 30 per cent for a factor of two increase or decrease in viscosity over the depth of the layer at wavelengths less than the layer depth there are dependences on both the rate of growth of viscosity and the surface boundary condition the latter dependency vanishing in the isoviscous case for greater viscosity variation several orders of magnitude topography and gravity kernels at all wavelengths are strongly affected viscosity increasing rapidly with depth can approximate a rigid lower boundary condition causing gravity kernels calculated with a free lower boundary condition to be negative for a significant range of source depths for very rapidly varying viscosity the sign of the perturbation pressure near the layer boundaries can be reversed from the isoviscous case resulting in topography kernels that are negative for some source depths

anomalous heat flow and geoid across the cape verde rise evidence for dynamic support from a thermal plume in the mantle summary we describe results from 200 heat flow measurements across the cape verde rise in the north atlantic they show that the heat flow through normal 125 myr old crust is 45 5 3 4mwm 2 close to the prediction for a lithospheric plate model heat flow increases towards the centre of the rise reaching a maximum of 16 4 mw m 2 above the normal oceanic value the concommitant geoid anomaly at the centre of the rise is 7 6 0 3 m and the depth anomaly is 1900 200 m the anomalous heat flow geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms lithospheric reheating models alone whether involving transient or sustained thinning of the lithosphere fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the cape verde rise axisymmetric convection models provide a better fit to the observed data and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle uplift caused by expansion of the solid lithosphere is of smaller importance the observed geoid bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2 4 times 1020 pa s and a coefficient of thermal expansion in the range 2 0 2 5 times 10 5 k 1 the effect of melting in the ascending plume is to buffer the heat flow anomalies in the case of the cape verde rise partial melting is minimal but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20 25 mw m 2

wavelet transform mapping of effective elastic thickness and plate loading validation using synthetic data and application to the study of southern african tectonics 1 we present a new wavelet transform method to map spatial variations in effective elastic thickness te and plate loading ratio f the method assumes a model of thin plate flexural isostasy to describe the mechanical response of the lithosphere to vertical loading in this model the rheological properties of the lithosphere are aggregated into the effective elastic thickness te of an equivalent thin plate overlying an inviscid fluid a number of methods have been developed to map spatial variations in te in an attempt to assess regional patterns of flexural strength our new method first obtains local coherence and local admittance through wavelet cross spectral analysis of surface topography and bouguer gravity anomaly wavelet coherence is used to obtain the local characteristic wavelength which is a function of both te and the degree of relative loading f of the plate by subsurface loads and surface loads loading is the combined effect of erosion sedimentation intrusion faulting and metamorphism wavelet admittance in a normalized form is used to resolve this f te ambiguity and maps are made of the spatial variations in te and f we carry out extensive tests of the wavelet method on simulated topography and bouguer gravity anomaly data that we generate through finite difference simulations of flexural isostasy with spectrally realistic loads and simple spatial variations in te these tests demonstrate that the wavelet inversion method is reasonably robust to uncertainties in loading and in crustal thickness and is able to recover te to within 25 50 of its correct value we apply the wavelet method to southern africa and recover estimates of te principally in the range 25 50 km in good agreement with existing estimates from forward modeling and fourier coherence analyses we find that our te estimates often exceed estimates of crustal thickness pointing to a strong upper mantle in parts of southern africa

reappraising elastic thickness variation at oceanic trenches we reassess the variation of elastic thickness as a function of lithospheric plate age using a global database of bathymetric and free air gravity profiles which are perpendicular to oceanic trenches as in many previous studies our starting point is the well known floating elastic plate model in order to remove the influence of short wavelength features not associated with lithospheric bending adjacent profiles from 10 myr bins have been stacked together to construct average profiles with standard deviations each average profile was then inverted in a two stage procedure first singular value decomposition was used to determine two unknown flexural parameters together with a regional slope and offset for any given elastic thickness this procedure was repeated for a range of elastic thicknesses second residual misfit was plotted as a function of elastic thickness and the global minimum was identified this two stage procedure makes no prior assumptions about magnitude of the load size of the bending moment or whether the elastic plate is broken continuous we obtained excellent fits between theory and observation for both bathymetric and gravity profiles from lithosphere with an age range of 0 150 ma the shape of the residual misfit function indicates the degree of confidence we have in our elastic thickness estimates the lower limit of elastic thickness is usually well determined but upper limits are often poorly constrained inverse modeling was carried out using a range of profile lengths 250 300 500 and 700 km in general our estimates show no consistent increase of elastic thickness as a function of plate age this surprising result is consistent with recent reassessments of elastic thickness beneath seamounts and implies either that elastic thickness is independent of plate age or that elastic thickness cannot be measured with sufficient accuracy to reveal such a relationship modeling of short free air gravity profiles 250 300 km does tentatively suggest that elastic thickness increases linearly from 5 to 10 km between 0 and 20 ma and from 10 to 15 km between 20 and 150 ma this variation roughly matches the depth to the 200 c isotherm which corresponds to an homologous temperature of 0 4 for wet peridotite unfortunately for longer profile lengths there is no temporal dependence and elastic thicknesses vary considerably for all plate ages bathymetric profile modeling yields similar results although uncertainties are larger

isostasy equivalent elastic thickness and inelastic rheology of continents and oceans the equivalent elastic thickness eet is used to estimate lithospheric strength expressed in response to loading by topography and subsurface loads the data on eet allow comparisons between different plates and detection of thermal events in oceans the eet corresponds to the mechanical core of the lithosphere i e a geotherm 400 600 c in continents the eet has no relation to any depth this has led to doubts in applicability of a unique approach to the continents and oceans and in the utility of estimates of eet for continents rheological data suggest that most rocks are inelastic in the long term 0 1 m y this requires interpretation of the eet in terms of real rheology we propose an analytical model that gives rheological interpretation of both the oceanic and continental eet it also allows estimates of the mechanical thickness of the lithosphere the eet depends upon three parameters geotherm age crustal thickness and flexural plate curvature any one of these values can be estimated if the others are known comparisons of model predictions with the observed eet suggest that most continental plates have a weak lower crust allowing mechanical decoupling between the upper crust and the mantle lithosphere such decoupling leads to strong reduction in the eet and thus can be easily detected flow of rocks in the weak lower crust may have a significant influence on the temporal evolution of relief mountain building erosion differences in the mechanical behavior of oceans and continents can be explained by domination of different parameters geotherm age has a major control in the oceans whereas in the continents crustal thickness is equally important additional local variations of the eet result from weakening by flexural stresses

lithological structural and geochemical characteristics of the mesoarchean târtoq greenstone belt southern west greenland and the chugach prince william accretionary complex southern alaska evidence for uniformitarian plate tectonic processes the mesoarchean t rtoq greenstone belt southern west greenland consists of tectonically imbricated slices of metamorphosed basalt gabbro peridotite and sedimentary rocks and is intruded by fel

geodynamic patterns of ophiolites and marginal basins in the indonesian and new guinea regions analysis of spatial temporal geological and geochemical patterns of ophiolites in the indonesian and new guinea region indicates a strong correlation with marginal basin development and closure the spatial distribution of ophiolites is mostly linked with marginal basin producing zones of oblique convergence and collision strain partitioning in these zones creates a series of ephemeral plate boundaries between several independently moving lithospheric blocks repeated disruption of the diffuse boundaries between the blocks by changes in plate motion and collision induced mantle extrusion creates space that is rapidly filled by new ocean basins in the upper plate of subduction zones suprasubduction zone ssz spreading of these basins is enhanced by episodic extrusion of asthenosphere escaping collisional suture zones various closure events and global plate motion changes are reflected in the temporal distribution of marginal basin and ophiolite ages most ophiolite slabs in the indonesian and new guinea region represent fragments of oceanic lithosphere with a subduction zone component as indicated by the common refractory petrochemistry of the mantle sequence and occurrence of boninite age and compositional heterogeneity may indicate that some ophiolite bodies are composite terranes collisions with buoyant lithosphere transfo parts of these ocean basins into ophiolites the connection between ophiolites and marginal basins is strongest where parts of actively spreading ssz basins are partially represented as ophiolites in collision zones many of the various tectonic models proposed for the origin and tectonic evolution of ophiolites are supported by plate interactions in the indonesian and new guinea region dewey bird 1970 silver smith 1983 moores et al 1984 searle stevens 1984 dilek moores 1990 harris 1992 dickinson et at 1996 however the geolo gical associations of these interactions remain poorly resolved allowing different parts of the region to be used as modern analogues for almost any tectonic scenario simple explanations based on plate kinematics alone fail to account for the increasing levels of complexity observed in the indonesian and new guinea region new ideas and supporting evidence for mantle extrusion mechanisms e g flower et at 2001 also rely heavily on descriptions of plate interactions in this region doglioni et at 1999 and raise new questions about how these processes may relate to the spatial and temporal pattern of ophiolites this paper provides a synthesis of the temporal and spatial patterns and tectonic associations of marginal basins and ophiolites throughout the indonesian and new guinea region it explores the relations between various plate boundary processes and ophiolite genesis and emplacement such as the role of strain partitioning trench rollback subduction polarity asymmetry astheno spheric flow and plate kinematics

inverse modelling of elastic thickness by convolution method the eastern alps as a case example an unconventional scheme is used to estimate the flexural rigidity or equivalently the elastic thickness of the lithosphere given the topography and gravity data the flexural rigidity is the parameter that governs the flexural response of the lithosphere in the frame of the thin plate flexure model the scheme is an alternative to the widely used calculation of admittance of topography sea floor or continental topography and gravity bearing some advantages which are explained in the paper the scheme involves the inversion of the gravity data in order to formulate a model of the crust mantle interface cmi undulations in a second step the flexure parameter is then evaluated from the relation between topography and cmi variations instead of calculating the admittance function using a spectral analysis a set of point load response functions are used in order to retrieve the optimal flexure parameter this has two main advantages instabilities of the numerical admittance evaluation at wavenumbers with low spectral energy in the topography are overcome and the analysis can be made over an area which is not necessarily rectangular as required for the spectral analysis the proposed method allows a higher space resolution of elastic thickness than any spectral method for validation the numerical strategy is applied to the situation of a realistic synthetic model where all inputs and outputs are known a priori finally the spatial variations of the elastic thickness are studied in an area across the eastern alps 4 2002 elsevier science b v all rights reserved

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

basement character and basin formation in gorontalo bay sulawesi indonesia new observations from the togian islands we present a new stratigraphy for the togian islands sulawesi and interpret the age character and evolution of gorontalo bay at its western end the bay is underlain by continental crust the central part is underlain by eocene to miocene oceanic and arc rocks although the area south of the togian islands could have continental crust of the banggai sula microcontinent thrust beneath this and the east arm ophiolite gorontalo bay was not a significant deep bathymetric feature before the miocene field relationships indicate a latest miocene to pliocene age for inception of the basin medium k to shoshonitic volcanism in the togian islands is not due to subduction but reflects crustal thinning and extension in the pliocene and pleistocene causing the underlying mantle to rise decompress and melt extension is continuing today and is probably the cause of volcanism at una una volcanic activity migrated west with time and volcanic products have been offset by dextral strike slip displacement along the balantak fault extension and subsidence was driven by rollback of the subduction hinge at the north sulawesi trench with a possible contribution due to flow of the lower crust 2011 the geological society of london

cenozoic plate interaction of the australia and philippine sea plates hit and run tectonics abstract recent studies in northwest new guinea have shown the presence of at least two marginal basins of different age both of which formed in back arc settings the older basin opened between the middle jurassic and early cretaceous a remnant of which is now preserved as the new guinea ophiolite its obduction started at 40 ma and it was finally emplaced on the australian margin at 30 ma the younger basin was active during the oligocene to middle miocene and was obducted in the early pliocene studies of the western edge of the philippine sea also reveal an important deformation of the philippine arc in the oligocene which hitherto has remained unexplained using information from these systems paleomagnetic results kinematic reconstructions and geochemistry of the supra subduction ophiolite we present a plate model to explain the region s eo oligocene development we suggest that an extensive portion of oceanic crust extended the australian plate a considerable distance north of the australian craton as australia began its steady 7 8 cm year northward drift in the early eocene this lithosphere was subducted thus the portion of the philippine sea plate carrying the taiwan philippine arc to its present site may have actually been in contact with the ophiolite now in new guinea and obduction led to deformation of the philippine sea plate itself neogene plate kinematics transported the deformed belt in contact with the sunda block in the late miocene and pliocene this interpretation has implications for the origin for the philippine sea plate and the potential incorporation of continental fragments against its boundaries

full waveform inversion of seismic reflection data from the forsmark planned repository for spent nuclear fuel eastern central sweden the swedish nuclear fuel and waste management company skb has been carrying out extensive studies at the planned repository for spent nuclear fuel at the forsmark site in the eastern part of central sweden since 2002 identification of subhorizontal to gently dipping seismic reflections is especially important since these may represent transport routes for radionuclides studies have shown that such reflections can be generated by water filled fracture zones that have a lower velocity than the surrounding bedrock lithological changes that is mafic sills may also be responsible for reflections in some cases at the forsmark site it is difficult to distinguish fracture zones from mafic sills in the standard reflection seismic processed sections however since mafic sills usually have a positive velocity contrast with the background velocity field compared to fractures zones that have a negative one the two possibilities could be differentiated if we could reconstruct the underground velocity field seismic full waveform inversion has the potential to perform this reconstruction allowing us to discriminate between fractures zones and mafic sills in this study we apply a 2 d waveform inversion code on crooked line data sets acquired at the forsmark site this implies we are dealing with a 3 d geometry we handle this problem by applying 3 d to 2 d coordinate projections first we perform a synthetic benchmark test with a similar geometry to that of the projected real data we test both amplitude and phase inversion and phase only inversion on the synthetic data the results show that the phase only inversion has fewer artefacts and is more stable after successful application on the synthetic data we apply the phase only waveform inversion on the real data the resulting velocity fields show more details compared with the starting model based on first arrival traveltime tomography time domain synthetic data sets generated from the final velocity fields show a good match between the early arriving phase of the real data and the synthetic data sets checkerboard tests are also applied on the final models for resolution checks and indicate good resolution down to 200 m in the central parts of the profiles based on a comparison between the kirchhoff pre stack depth migrated sections and the final waveform inversion results we conclude that the reflections investigated most likely are generated at low velocity fracture zones

elastic thickness of the lithosphere in the central canadian shield we have used the coherence between bouguer gravity and topography to determine the elastic thickness t e of the lithosphere in central canada in order to improve the resolution we have estimated the spectra with the maximum entropy method the variations in elastic thickness delineate three main regions 1 the interior platform where t e varies between 25 and 50 km 2 the shield includes several high rigidity cores in particular in the trans hudson orogen and in the hearne province where t e 80km 3 the hudson bay basin has an elastic thickness between 50 and 70 km with higher values in the eastern part the results suggest that t e is controlled by the thermal regime in the platform there is no difference in t e between the proterozoic and archean provinces of the shield in the platform the elastic thickness is lowest where sediments are thickest indicating that low rigidity has amplified the subsidence of the basin

effect of windowing on lithosphere elastic thickness estimates obtained via the coherence method results from northern south america 1 the effective elastic thickness te of the lithosphere is a parameter that describes the flexural strength of a plate a method routinely used to quantify this parameter is to calculate the coherence between the two dimensional gravity and topography spectra prior to spectra calculation data grids must be windowed in order to avoid edge effects we investigated the sensitivity of te estimates obtained via the coherence method to mirroring hanning and multitaper windowing techniques on synthetic data as well as on data from northern south america these analyses suggest that the choice of windowing technique plays an important role in te estimates and may result in discrepancies of several kilometers depending on the selected windowing method te results from mirrored grids tend to be greater than those from hanning smoothed or multitapered grids results obtained from mirrored grids are likely to be over estimates this effect may be due to artificial long wavelengths introduced into the data at the time of mirroring coherence estimates obtained from three subareas in northern south america indicate that the average effective elastic thickness is in the range of 29 30 km according to hanning and multitaper windowed data lateral variations across the study area could not be unequivocally determined from this study we suggest that the resolution of the coherence method does not permit evaluation of small i e 5 km local te variations however the efficiency and robustness of the coherence method in rendering continent scale estimates of elastic thickness has been confirmed

cenozoic palaeogeographic evolution of sulawesi and borneo abstract sulawesi and borneo are located in the middle of the indonesian archipelago an area which has been extremely tectonically active throughout the cenozoic this paper compiles current knowledge on the geology and palaeogeography of this complex region and includes the first attempt to synthesise palaeogeographic data onto plate tectonic reconstructions construction of palaeogeographic maps helped to identify some of the strengths and weaknesses of current plate tectonic models for the region thereby highlighting areas where further geological research is required palaeogeographic maps presented using plate tectonic reconstructions as a base illustrate the evolution of borneo and sulawesi and highlight important changes in the environment during the cenozoic the tertiary geological history of eastern borneo and sulawesi is inextricably linked to the progressive accretion of continental and oceanic material from the east onto the eastern margin of sundaland and to the resultant development of volcanic arcs large tracts of western sulawesi eastern borneo the east java sea and the makassar straits formed an extensive basinal area throughout much of the tertiary implications for plate tectonics exploration for natural resources and biogeography are discussed from the palaeogeographic reconstructions of sulawesi and borneo

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

the formation of tharsis on mars what the line of sight gravity is telling us line of sight los spacecraft acceleration profiles from the radio science experiment and topography from the mars orbiter laser altimeter mola instrument of the mars global surveyor mgs are analyzed to estimate the effective elastic thickness te for various regions of tharsis we identify a buried basin flanking the thaumasia highlands at the southeastern margin of tharsis assuming that this basin results from lithospheric flexure from surface loading by the thaumasia highlands we fit los profiles across the feature with a thin shell elastic flexure model and find the mountain belt to reflect a value of te 20 km consistent with a noachian formation age we also determine admittances from los profiles for five regions across tharsis and fit them with theoretical admittances calculated using the flexural model crater density surface density and predominant surface age are found to vary systematically across tharsis while te does not the highest surface density and lowest te values are obtained for the western portion of tharsis where crater densities are lowest our results imply the majority of the topographic rise was emplaced within the noachian irrespective of the surface ages topographic loading and resurfacing i e volcanic activity persisted into the amazonian while becoming increasingly confined to the western margin where the youngest surface ages are found and the eruptive style transitioned from effusive volcanism to shield forming volcanism as te increased

compensation of the ontong java plateau by surface and subsurface loading gravity data over ontong java plateau reveal deep crustal structure supporting the plateau s topography at long wavelengths 500 km new shipboard gravity and bathymetry profiles indicate local isostatic compensation at intermediate wavelengths 250 500 km however bouguer anomalies have large amplitudes too high to be explained by models of airy isostasy or volcanic loading on the surface of an elastic lithospheric plate this finding is also evident in admittance functions generated from maps of bathymetry and satellite derived bouguer anomalies which show high values at intermediate wavelengths in addition coherence between two dimensional bouguer gravity and bathymetry spectrums decreases to zero beginning at intermediate wavelengths two possible models can explain the high admittance and low coherence at intermediate wavelengths the first model explains the high admittance by large scale lithospheric folding and explains the low coherence by sedimentation and erosion that is uncorrelated with the igneous crustal structure lithospheric folding might result from tectonic compression imposed on ontong java by the solomon arc and trench system the second model involves a multiple stage accretion history with surface magmatism on a thin elastic plate and magmatic underplating beneath a thick plate the thick plate during underplating is required to explain the high admittance values at intermediate wavelengths while decorrelation between underplating and the surface loaded magmas can explain the low coherence the thick plate also suggests that the lithosphere cooled appreciably since the first magmatic stage and therefore magmatic underplating occurred at a significantly later time than the initial eruption calculations predict that the underplated material could comprise as much as 50 of the total crustal volume suggesting that the later eruptive stage s may have been as or more voluminous than the first

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

emplacement of the east sulawesi ophiolite evidence from subophiolite metamorphic rocks abstract a newly discovered high temperature metamorphic sole at the base of the extensive east sulawesi ophiolite in sulawesi indonesia yields new structural and petrological data with which to constrain the emplacement history the metamorphic sole is composed of a thin sequence of garnet and epidote amphibolite and basal greenschist metaclastics east dipping tectonite fabrics in the metamorphic sole amphibolite are coincident with those in subjacent basement mica schist and superjacent peridotite and indicate orthogonal east to west emplacement of the ophiolite in central sulawesi metabasite in the upper levels of the sole has morb geochemical characteristics was recrystallized at temperatures of about 700 c and yielded k ar radiometric ages of approximately 30 ma initial detachment of the ophiolite probably occurred at a ridge segment transform intersection in a back arc basin at the south east margin of sundaland detachment closely preceded nw directed overthrusting and orthogonal obduction on to the sundaland continental margin in the oligocene sinistral movement on a paleo transform boundary now the lawanopo fault may have facilitated obduction although the westernmost segments of ophiolite may represent younger more proximal slices of a marginal basin those further east probably represent successively older more distal slices of underthrust indian ocean back arc or forearc

the long wavelength admittance and effective elastic thickness of the canadian shield the strength of the cratonic lithosphere has been controversial on the one hand many estimates of effective elastic thickness te greatly exceed the crustal thickness but on the other the great majority of cratonic earthquakes occur in the upper crust this implies that the seismogenic thickness of cratons is much smaller than te whereas in the ocean basins they are approximately the same leading to suspicions about the large te estimates one region where such estimates have been questioned is the canadian shield where glacial isostatic adjustment gia and mantle convection are thought to contribute to the long wavelength undulations of the topography and gravity to date these have not been included in models used to estimate te from topography and gravity which conventionally are based only on loading and flexure here we devise a theoretical expression for the free air gravity topography admittance that includes the effects of gia and convection as well as flexure and use it to estimate te over the canadian shield we use wavelet transforms for estimating the observed admittances after showing that multitaper estimates which have hitherto been popular for te studies have poor resolution at the long wavelengths where gia and convection predominate compared to wavelets our results suggest that te over most of the shield exceeds 80 km with a higher te core near the southwest shore of hudson bay this means that the lack of mantle earthquakes in this craton is simply due to its high strength compared to the applied stresses

global gravity bathymetry and the distribution of submarine volcanism through space and time 1 the seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin relatively few of these features 0 1 however have been dated and so little is known about their tectonic setting one parameter that is sensitive to whether a seamount formed on near or far from a mid ocean ridge is the elastic thickness te which is a proxy for the long term strength of the lithosphere most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of te and then comparing the calculated and observed gravity anomaly the problem with such an approach is that bathymetry data are usually limited to single beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape we therefore use the satellite derived gravity anomaly to predict the bathymetry for different values of te by comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the wessel global seamount database we have obtained 9758 te estimates from a wide range of submarine volcanic features in the pacific indian and atlantic oceans comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate te and its upper and lower bounds te at sites where there is both a sample and crustal age show considerable scatter however and there is no simple relationship between te and age nevertheless we are able to tentatively assign a tectonic setting to each te estimate the most striking results are in the pacific ocean where a broad swath of on ridge volcanism extends from the foundation seamounts and ducie island easter island ridge in the southeast across the equator to the shatsky and hess rises in the northwest interspersed among the on ridge volcanism are flank ridge and off ridge features the indian and atlantic oceans also show a mix of tectonic settings off ridge volcanism dominates in the eastern north atlantic and northeast indian oceans while flank ridge volcanism dominates the northeastern indian and western south atlantic oceans we have been unable to assign the flank ridge and off ridge estimates an age but the on ridge estimates generally reflect we believe the age of the underlying oceanic crust we estimate the volume of on ridge volcanism to be 1 1 106 km3 which implies a mean seamount addition rate of 0 007 km3 yr 1 rates appear to have varied through geological time reaching their peak during the late early cretaceous and then declining to the present day

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

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

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