seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

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

can the aluminum and titanium contents of ocean sediments be used to determine the paleoproductivity of the oceans in the central equatorial pacific settling particles collected with sediment traps have al ti values greater than those of a typical crustal source the al ti values of the sediment trap material are not well correlated with either the flux of organic carbon or with total particle mass suggesting that this ratio cannot be used as a simple index of paleoproductivity however the al ti values of some trap samples are more than 3 times the ratio for typical crustal materials indicating a noncrustal source of excess al numerical analysis using linear programming indicates that the excess al is associated with opal and that this biological component carries as much as 1200 ppm of al because of the low particulate fluxes of lithogenic material in the equatorial pacific and the relatively high opal content of equatorial sediment trap material opal appears to be the predominant conveyor of al to the equatorial sediments our sediment trap al ti values are similar to ratios previously measured in equatorial sediments suggesting that measurements of this element pair in ocean sediments may be an indicator of opal rain to the seafloor the particle reactive nature of al and ti means their ratios in sediments may be relatively insensitive to opal preservation or other diagenetic processes however the al ti values of raining particles are highly sensitive to the relative proportion of opal and lithogenic material where opal lithogenic ratios are less than 20 the al ti values are indistinguishable from a crustal source ratio in addition the amount of excess al associated with opal in settling particles may be a function of the concentration of al in the water column and of the processes that fractionate dissolved al and ti these issues must be evaluated before the al ti values of ocean sediments can be used as a reliable proxy of opal flux to the seafloor

s wave velocity structure of the canadian upper mantle abstract we investigate mantle structure beneath the canadian landmass using 500 surface wave waveforms recorded at stations of the canadian national seismic network and global seismic network the data are processed using the method of partitioned waveform inversion and afford a comprehensive sampling of s velocity structure beneath the north american craton and adjacent orogenic belts our study thereby extends results from previous regional s wave studies north of the coterminous us and provides a characterization of mantle structure beneath canada which is complementary to the body wave model of grand j geophys res 99 1994 11591 the resulting 3d velocity model is well resolved to a depth of 400 km over most of the region and displays two prominent large scale anomalies a high velocity structure beneath the canadian shield and platform associated with the cratonic keel and pronounced low velocities beneath the cordillera the contrast between these features is strong up to 10 and at more southerly latitudes is sharp with the transition occurring over distances of 600 km or less laterally coherent high velocities persist to depths of ca 250 km beneath the north american craton which we interpret to represent the base of the continental keel smaller scale variations are observed within the cratonic mantle but in general show little correlation with surface geological provinces moderately low velocities beneath the st lawrence valley region and labrador may be related to intracontinental volcanism and rifting respectively

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

a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

mechanical controls on the deformation of continents at convergent margins abstract spatial variations in the rigidity of continental plates expressed in terms of an effective elastic thickness t e can have a profound influence on the style of deformation of ocean continent convergent margins depending on the spatial distribution of t e strains related to plate boundary forces can become concentrated where t e is small we calculate t e and t e anisotropy in western canada by using and comparing the wavelet and multitaper coherence methods in addition to a nearly stepwise change in t e from the cordillera to the craton we show that weak axes of t e anisotropy are parallel with most compressive directions of horizontal stress components and fast axes of upper mantle seismic anisotropy maxima in the magnitude of t e anisotropy are spatially correlated with the locations of most earthquakes and the locations and directions of maxima in electrical anisotropy and conductivity the pattern of brittle failure and seismicity is explained as a response to plate boundary forces acting on a plate of variable rigidity and is expected to induce the observed mechanical anisotropy and enhance the flow of crustal fluids resulting in locally high electrical conductivities our combined results thus suggest for the first time that the elastic properties of continental lithosphere have a leading order influence on the deformation and evolution of convergent plate boundaries

resistance to mantle flow inferred from the electromagnetic strike of the australian upper mantle seismic anisotropy is thought to result from the strain induced lattice preferred orientation of mantle minerals especially olivine1 2 owing to shear waves propagating faster along the a axis of olivine crystals than along the other axes this anisotropy results in birefringence or shear wave splitting 3 which has been investigated in numerous studies1 4 although olivine is also anisotropic with respect to electrical conductivity5 with the a axis being most conductive few studies of the electrical anisotropy of the upper mantle have been undertaken and these have been limited to relatively shallow depths in the lithospheric upper mantle6 7 theoretical models of mantle flow have been used to infer that for progressive simple shear imparted by the motion of an overriding tectonic plate the a axes of olivine crystals should align themselves parallel to the direction of plate motion8 9 here however we show that a significant discrepancy exists between the electromagnetic strike of the mantle below australia and the direction of present day absolute plate motion10 we infer from this discrepancy that the a axes of olivine crystals are not aligned with the direction of the present day plate motion of australia indicating resistance to deformation of the mantle by plate motion

spatial variations of effective elastic thickness over the ninetyeast ridge and implications for its structure and tectonic evolution we present new data on the strength of oceanic lithosphere along the ninetyeast ridge ner from two independent methods spectral analysis bouguer coherence using the fan wavelet transform technique and spatial analysis flexure inversion with the convolution method the two methods provide effective elastic thickness t e patterns that broadly complement each other and correlate well with known surface structures and regional scale features furthermore our study presents a new high resolution database on the moho configuration which obeys flexural isostasy and exhibit regional correlations with the t e variations a continuous ridge structure with a much lower t e value than that of normal oceanic lithosphere provides strong support for the hotspot theory the derived t e values vary over the northern higher t e similar to 10 20 km central anomalously low t e similar to 0 5 km and southern low t e similar to 5 km segments of the ner the lack of correlation of the t e value with the progressive aging of the lithosphere implies differences in thermo mechanical setting of the crust and underlying mantle in different parts of the ner again indicating diversity in their evolution the anomalously low t e and deeper moho similar to 22 km estimates of the central ner between 0 5 degrees n and 17 degrees s are attributed to the interaction of a hotspot with the wharton spreading ridge that caused significant thermal rejuvenation and hence weakening of the lithosphere the higher mechanical strength values in the northern ner north of 0 5 degrees n may support the idea of off ridge emplacement and a relatively large plate motion at the time of volcanism the low t e and deeper moho similar to 22 km estimates in the southern part south of 17 degrees s suggest that the lithosphere was weak and therefore younger at the time of volcanism and this supports the idea that the southern ner was emplaced on the edge of the indian plate c 2013 elsevier b v all rights reserved

architecture of the crust and uppermost mantle in the northern canadian cordillera from receiver functions the northern canadian cordillera ncc is an active orogenic belt in northwestern canada characterized by deformed autochtonous and allochtonous structures that were emplaced in successive episodes of convergence since the late cretaceous seismicity and crustal deformation are concentrated along corridors located far 200 to 800 km from the convergent plate margin proposed geodynamic models require information on crust and mantle structure and strain history which are poorly constrained we calculate receiver functions using 66 broadband seismic stations within and around the ncc and process them to estimate moho depth and p to s velocity ratio vp vs of the cordilleran crust we also perform a harmonic decomposition to determine the anisotropy of the subsurface layers from these results we construct simple seismic velocity models at selected stations and simulate receiver function data to constrain crust and uppermost mantle structure and anisotropy our results indicate a relatively flat and sharp moho at 32 2 km depth and crustal vp vs of 1 75 0 05 seismic anisotropy is pervasive in the upper crust and within a thin 10 15 km thick sub moho layer the modeled plunging slow axis of hexagonal symmetry of the upper crustal anisotropic layer may reflect the presence of fractures or mica rich mylonites the subhorizontal fast axis of hexagonal anisotropy within the sub moho layer is generally consistent with the se nw orientation of large scale tectonic structures these results allow us to revise the geodynamic models proposed to explain active deformation within the ncc

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

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

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

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

abyssal sediment erosion in the central indian basin evidence from radiochemical and radiolarian studies abstract radiochemical analyses of three spade cores collected around 78 e between 10 s and 12 s from the abyssal depths in the central indian basin yield an average accumulation rate of 2 mm ka the ratios of 230th flux in the sediments to its production rate in the overlying water column fa fp are extremely low 0 2 this fact and the absence of collosphaera invaginata first appearance datum 150 200 ka an index radiolarian species of neogene radiolarian zone 1 indicate intense erosion and lateral transport of younger sediments from this region the radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent antarctic bottom water entering the central indian basin through the northern saddles of the ninety east ridge

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

linearized inverse scattering of teleseismic waves for anisotropic crust and mantle structure 1 theory 1 in this study we examine the recovery of vertically varying anisotropic crust and upper mantle structure using converted teleseismic waves recorded on three component seismograms from individual broadband stations our analysis is cast in terms of inverse scattering theory for a one dimensional medium and begins with a derivation of the plane wave green s function for a homogeneous medium exhibiting arbitrary elastic anisotropy this green s function is employed within the single scattering approximation to derive formulae that relate the amplitude of the scattered wave in time to perturbations in material properties at corresponding depths inclusion of seismograms from multiple events representing a range of azimuths and incident angles leads to the construction of a linear system of equations that is readily solved using singular value decomposition a useful by product of this amplitude versus slowness approach is the identification of simple and compact expressions for linearized reflection and conversion coefficients in anisotropic media that are accurate for small contrasts at near vertical angles of incidence we demonstrate the accuracy of the linearized p to s transmission coefficient for an idealized upper mantle model in a companion paper we will examine the application of this approach to field data recorded at stations of the canadian national seismograph network

estimating the mechanical anisotropy of the iranian lithosphere using the wavelet coherence method abstract we calculated anisotropic wavelet coherence between bouguer anomaly and topography in order to map the anisotropy of the effective elastic thickness of the iranian lithosphere t e an orthotropic elastic plate model is used for inverting the anisotropic wavelet coherence to compute the mechanical anisotropy through the weak axis of the t e anisotropy of the t e weak axis and the strength of the anisotropic parameter namely the anisotropic coherence effect over the study area are estimated by restricting the rotated morlet wavelet fan wavelet geometry over an azimuthal range of 90 large scale t e variations have been shown to be associated with phenomena such as mountain belts subduction zones craton boundaries fault zones and seismogenic regions although the correlation between the major tectonic features of the iranian lithosphere and the distribution of the t e weak axis is not general or precise in some regions such as the central iran blocks and the alborz kopeh dagh zagros and makran orogenic belts the weak axis has a uniform or slowly varying pattern which changes over their boundaries a perpendicular alignment between seismic anisotropy measurements in iran and the t e weak directions suggests a lithospheric origin for anisotropy the correlation between averaged stress directions and the weak axis of the t e in iran indicates that the present day stress field and the fossil strain are still related correlation between these factors suggests vertically coherent deformation of the lithosphere in iran resulting from the multiply convergent orogenic processes the complex mechanical anisotropy pattern of the iranian lithosphere results from the interaction of many pre existent structures which dominantly control the mechanical anisotropy of the lithosphere

biogenic flux of al to sediment in the central equatorial pacific ocean evidence for increased productivity during glacial periods we examined the flux of al to sediment accumulating beneath the zone of elevated productivity in the central equatorial pacific ocean along a surface sediment transect at 135 w as well as downcore for a 650 kyr record at 1 3 n 133 6 w across the surface transect a pronounced broadly equatorially symmetric increase in al accumulation is observed relative to ti with al ti ratios reaching values 3 4 times that of potential detrital sources the profile parallels biogenic accumulation and the modeled flux of particulate 234th suggesting rapid and preferential adsorptive removal of al from seawater by settling biogenic particles normative calculations confirm that most al is unsupported by the terrigenous fraction the observed distributions are consistent with previous observations of the relative and absolute behavior of al and ti in seawater and we can construct a reasonable mass balance between the amount of seawater sourced al retained in the sediment and the amount of seawater al available in the overlying column the close tie between al ti and biogenic accumulation as opposed to concentration emphasizes that biogenic sedimentary al ti responds to removal transport phenomena and not bulk sediment composition thus in these sediments dominated by the biogenic component the bulk al ti ratio reflects biogenic particle flux and by extension productivity of the overlying seawater the downcore profile of al ti at 1 3 n displays marked increases during glacial episodes similar to that observed across the surface transect from a background value near al ti of average upper crust the excursions in al ti are stratigraphically coincident with maxima in both bulk and caco3 accumulation and the excess al appears to not be preferentially affiliated with opaline or organic phases consistent with the similar behavioral removal of al and 234th the latter of which responds to the total particle flux the al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits these results collectively indicate that 1 al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater therefore the common tenet that al is dominantly associated with terrestrial particulate matter and the subsequent use of al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment needs to be reevaluated 2 the al ti ratio in biogenic sediment can be used to trace the productivity of the overlying water providing a powerful new paleochemical tool to investigate oceanic response to climatic variation 3 the close correlation between the al ti productivity signal and carbonate maxima downcore at 1 3 n suggests that the sedimentary carbonate maxima in the central equatorial pacific ocean record increased productivity during glacial episodes

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

distribution of aluminium in waters of the north east atlantic 25 n to 35 n abstract dissolved al concentrations from the ne atlantic between 25 n and 35 n off the coast of nw africa range from 16 nm to 32 nm in a well mixed surface layer extending to 150 m depth increasing towards the maximum of the saharan dust plume mediterranean outflow water is detected as a high al anomaly corresponding to the salinity signal of the mediterranean water other than in surface and mediterranean waters al concentrations are invariant with depth but increase from 6 nm to 11 nm towards the continental margin the magnitude of the dissolved al signal in the surface layer is compatible with dissolution of al from the highest of present estimates of dust input to this area comparison with data from other areas indicates that al cannot be cycled through the oceans in parallel to silica as has been previously suggested

aluminium in the northwest atlantic abstract a series of profiles between woods hole and bermuda has been measured on board ship for al the extrema in the upper waters can be explained by the accumulation of aeolian inputs in the thin summer mixed layer the depletion of the underlying winter water by scavenging during the spring bloom and the enrichment of the subtropical mode 18 c water by aeolian accumulation during winter and local convection before the spring bloom the pronounced minimum at mid depth is associated with the antarctic intermediate water and the underlying labrador sea med water complex presumably the aaiw and the med water are depleted by the ageing effect on this short residence time element during their transit to the area the lsw is formed from surface waters containing a large component of arctic waters extremely depleted in al advected south in the east greenland current the linear increase to the bottom is caused by mixing of enriched lower north atlantic deep water with the depleted bottom water and lsw cores the enrichment in the deep water appears to be produced by the entrainment of high al waters from the north european shelf by the gyre in the norwegian greenland sea and its injection into the denmark straits overflow waters in a manner recently described for the radiocaesium releases from the windscale reprocessing plant on the irish sea an additional contribution probably comes from the shelf north of iceland

scavenged excess aluminum and its relationship to bulk titanium in biogenic sediment from the central equatorial pacific ocean we present results from chemical analyses of al and ti in surface sediment sampled along two cross equator latitudinal transects at 135w and 140w in the central equatorial pacific ocean although traditionally both al and ti are considered to reside exclusively within terrigenous phases in marine sediment these sediments present extremely high al ti ratios that are several times that of average shale and other potential crustal sources a sharp maximum in al ti is observed slightly south of the equator where sedimentary bulk accumulation rate bar is also highest reflecting elevated productivity in the overlying water caused by surface water divergence bulk al ti decreases sharply away 2 latitude to near crustal values at 4 north and south the latitudinal profiles of al ti are entirely unrelated to the concentration of the biogenic components as well as to the absolute accumulation of al and ti these results indicate the presence of a significant scavenged component of al sourced directly from seawater during particle settling the data from the two transects analytically and oceanographically confirms our earlier work that was based on the single 135w transect calculations of alexcess indicate that the highest al ti ratios correspond to 50 of the total al being unsupported by the small amount of terrigenous phases present these results are consistent with previous and ongoing studies of biogenic sediment suspended particulate matter and sediment trap material quantitative use of al as an index of terrigenous material may therefore lead to an overestimation by a factor of two of the true terrigenous load in marine sediment sedimentary rock and settling particles because bulk al ti appears to respond to sedimentary bar which in biogenic regimes is linked to surface water productivity downcore records of al ti in biogenic sediment may track productivity changes through time such al ti proxy records may be applicable in sediment of all ages unlike radionuclide tracers which are limited by radioactive decay to use over the past hundreds of kyr

anisotropy of the flexural response of the lithosphere in the canadian shield 1 we show that the two dimensional coherence between topography and bouguer gravity is anisotropic throughout most of the canadian shield excepted hudson bay basin we interpret the axis of maximum coherence as the direction where the lithosphere is weakest in the southeastern shield and the appalachians the azimuthally averaged coherence cannot be fitted by the thin elastic plate model in the other regions it yields the effective elastic thickness the average coherence increases in the direction perpendicular to the main tectonic boundaries the axis of high electrical conductivity and the seismic fast axis this consistency suggests that the same stress field affected the entire lithosphere during the last tectonic event

simplified analysis of horizontal stresses in a buttressed forearc sliver at an oblique subduction zone forearc slivers are commonly present at obliquely convergent margins such slivers are often buttressed at their leading edges but no quantitative model of the buttress effect exists i consider the horizontal stresses in a buttressed sliver using a simple plan view two dimensional elastic model the driving force created by the subducting plate is approximated by a body force in the direction of plate convergence an airy stress function solution relates the stress field to the boundary and body forces that allows the relative importance of different forces driving and resisting the sliver to be evaluated oblique subduction can easily cause arc parallel compression in the buttressed sliver especially near the leading edge because the arc parallel component of the driving force by the subducting plate is integrated over a much larger distance than its arc normal component for the southwest kuril arc the stress field predicted by the simple model agrees with that inferred from geological structures it is possible that the model can also explain the stress field of the cascadia forearc as constrained by earthquake focal mechanisms and in situ stress measurements

tectonic inheritance of the indian shield new insights from its elastic thickness structure abstract a new evaluation of the elastic thickness te structure of the indian shield derived from isotropic fan wavelet methodology documents spatial variations of lithospheric deformation in different tectonic provinces correlated with episodic tectono thermal events the te variations corroborated by shear velocity crustal thickness and seismogenic thickness reveal the heterogeneous rheology of the indian lithosphere the thinned attenuated lithosphere beneath peninsular india is considered to be the reason for its mechanically weak strength

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

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

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

body wave tomography of western canada abstract in this study we have produced p and s wave velocity models for western canada using 23 420 delay times measured on vertical component seismograms and 15 805 delay times measured on transverse component seismograms respectively from a range of permanent and temporary networks resolution is best in southwestern british columbia and along the canoe northwestern alberta southern yukon and northwest territories and batholiths northwestern bc arrays where the station density is the highest and fair elsewhere we focus our attention on two distinct features 1 the transition from phanerozoic to cratonic mantle in northwestern canada and 2 the complex tectonic environment at the northern terminus of the cascadia subduction zone where the plate boundary changes from convergent to transform we find that the main transition from phanerozoic to cratonic mantle in northwestern canada occurs at the cordilleran deformation front and represents a sharp jump in seismic velocity from 2 to 2 over a distance of 50 km in northern cascadia we have imaged and characterized the signature of the subducting juan de fuca plate and observed evidence of subduction beyond the northern edge of the slab we also demonstrate that the anahim hotspot track is underlain by a 2 low velocity zone possibly extending to 400 km beneath nazko cone that appears to be the source of volcanism in this area consequently we associate the source of magmatism in this area to a mantle scale rather than lithospheric scale process

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

elastic thickness mechanical anisotropy and deformation of the southeastern tibetan plateau abstract the southeastern tibetan plateau se tibet located between the eastern himalayan syntaxis and sichuan basin is a heterogeneous structure combining different tectonic regimes it encompasses a gently sloping long wavelength margin and is characterized by intense deformation and active seismicity mechanical strength of the lithosphere is one of the parameters that controls deformation and evolution of any region in particular anisotropy of the strength is important for understanding the relation between tectonic processes and lithosphere deformation as a proxy the effective elastic thickness of the lithosphere t e can be used to study both spatial distribution and anisotropy of the lithospheric strength here we use the fan wavelet coherence method to determine variations of t e and its anisotropy based on the topography and bouguer gravity anomaly data the coherence analysis shows that low t e 5 30 km and high t e anisotropy are widespread in se tibet indicating a weak and highly mechanical anisotropic lithosphere this also implies that the style of lithospheric deformation is continuous and may be dominated by lower crustal flow compared with the crustal stress and lithospheric strain indicators we find that the t e anisotropy is predominantly governed by the present day tectonic stress in se tibet this may indicate a post collision compression tectonics within the plateau due to the india eurasia plate convergence and a resistance by the rigid sichuan basin however the anisotropy reflects extension stresses in approximately wnw ese direction in yunnan and northern indochina off the plateau which may relate to the burma plate subduction processes we infer that the present day tectonic stresses and mantle processes associated with the india eurasia collision continuously modify and rework the mechanical structure and both govern the lithospheric deformation in se tibet

control of lithospheric inheritance on neotectonic activity in northwestern canada lithospheric inheritance is thought to affect the location and reactivation of tectonic structures through successive cycles of supercontinent formation and dispersal however its relation to neotectonic activity remains unclear in northwestern canada abundant seismicity throughout the northern canadian cordillera ncc is geographically confined by several crustal scale boundaries yet its southern extent terminates abruptly along the inferred westward extension of a late cretaceous rifted margin boundary called the liard transfer zone ltz we use seismic data to show that the uppermost mantle beneath the cordillera exhibits a sharp north south contrast in fabric across the ltz south of the ltz fast axes of seismic wave propagation align closely with the lithospheric mantle fabric orientation of the adjacent canadian shield north of the ltz fast axes are reoriented subparallel to the motion of the pacific plate and follow the strike of the large dextral strike slip tintina and denali faults we attribute changes in anisotropic delay times across the tintina and denali faults to localized shear within the lithosphere this implies that the crust and lithospheric mantle remained mechanically coupled during shearing we propose that the contrast in uppermost mantle structure across the ltz reflects a change in the nature and origin of the lithospheric mantle from inherited rifted margin structures which affects the stability of the lithosphere and limits the extent of seismic activity within the ncc these results indicate that neotectonic activity in modern cordilleras is controlled in part by inherited upper mantle structures

excess aluminum in deep sea sediments of the central indian basin abstract the aluminum concentration in the marine sediments has long been considered an indicator of terrigenous abundance the al ti values 48 5 of the central indian basin subsurface siliceous sediment are up to three times that of post archean australian shale paas the estimation of structurally unsupported al al ex from six sediment cores suggests that an average of 40 of total al is from noncrustal sources higher al ti or al ex values are often but not exclusively associated with abundant volcanic unaltered glass shards electron microprobe analysis of 60 of these glass shards reveals that they are rhyolitic in composition and of youngest toba 74 ka origin from northern sumatra these glass shards have very high al ti ratio of 175 mean al ti values are also above the paas in sediment where glass shards are absent which could be due to scavenging of dissolved al by biogenic components

effective elastic thickness and mechanical anisotropy of south china and surrounding regions abstract south china and surrounding regions extend from the eastern tibetan plateau through the tectonically stable sichuan basin and the broad mesozoic magmatic and fold belt to the trench arc basin system in the western pacific which provide an ideal place to study deformation of the continental lithosphere under long term magmatism and oceanic subduction we obtained the effective elastic thickness t e and its anisotropy of south china and surrounding regions from the analysis of coherence between topography and satellite gravity using wavelet methods the t e values of the study area vary from 2 to 75 km with relatively low t e values 30 km along the tectonic boundaries the north south gravity lineament nsgl and seismic zones and in the regions with high surface heat flow the evenly low t e values in the lower yangtze region and the cathaysia block can be attributed to the long lived subduction of the paleo tethys and the paleo pacific ocean basins beneath the south china block scb the nsgl in the scb may separate the unmodified high t e and thermally weakened low t e continental lithosphere due to oceanic subduction despite different distances to the tectonic boundaries earthquakes occur more frequently in regions with t e values of 10 30 km implying strain concentration in the low t e regions a positive correlation between seismic activity and the magnitude of t e anisotropy suggests that a highly anisotropic mechanical structure will promote strain localization and brittle failure in the lithosphere the poor correlation between the weak axis of t e anisotropy and the dynamic indicators of the present tectonic regime the shear wave splitting direction the maximum horizontal compressive stress direction confirms that t e anisotropy mainly reflects tectonic inheritance of the continental lithosphere

deep electrical structure of the northern cascadia british columbia canada subduction zone implications for the distribution of fluids long period magnetotelluric data have been used to image the deep electrical structure of the cascadia subduction zone in british columbia canada zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab a shallow zone of high conductivity beneath vancouver island is likely due to fluids that are trapped above the subducting plate east of this structure is a conductive 0 03 s m forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized a free fluid phase is required to account for this enhanced conductivity elevated conductivities are observed in the upper mantle throughout the backarc 0 01 s m and strongly support the hypothesis of a shallow convecting asthenosphere this enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals or by a few percent partial melting 4

new occurrence of youngest toba tuff in abyssal sediments of the central indian basin abstract volcanic glass and pumice found in siliceous abyssal sediments of the central indian basin south of the equator have previously been assigned various origins including intra basin volcanism indonesian arc and krakatau rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the youngest toba tuff erupted at 74 ka from northern sumatra the correlation extends the distribution of coarse 63 m glass shards some 1500 km south of the previously known fallout zone and into the southern hemisphere reaching latitudes of ca 14 s this provides evidence for bi hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption such dispersal could have facilitated the global impact of the eruption

geophysical evidence on segmentation of the tancheng lujiang fault and its implications on the lithosphere evolution in east china abstract the north south trending tancheng lujiang tanlu fault belt extends from northeast china to the dabie sulu orogenic belt for a length of more than 3000 km this fault belt probably has close links with the lithosphere evolution seismic activity and mineral resource concentration in east china surface geological mapping and studies on sedimentation and basin formation have indicated segmentation at the southern middle and northern domains of the fault here we employ geophysical constraints to evaluate these fault segments unlike previous geophysical studies focused on laterally varying crust mantle seismic velocity structure across the fault in this study we have integrated a variety of geophysical data sets such as crustal p wave velocity earthquake occurrence and released seismic energy seismogenic layer thickness surface heat flow and geothermal field to understand the deep structure and strength of the lithosphere along the tanlu segmented fault belt the results demonstrate remarkable crustal scale north to south segmentation this major fault the geophysical evidence and some geochemical constraints suggest that the tanlu fault belt probably served as a channel for melt and fluid percolation and exerted a significant control on the lithosphere evolution in east china

regional electrical resistivity structure of the southern canadian cordillera and its physical interpretation the regional geoelectric crustal structure of the southern and central canadian cordillera of western canada is interpreted from the inversion of magnetotelluric data along five profiles crossing the physiographic morphogeological belts with emphasis on the intermontane and omineca belts decomposition of the tensor impedance response estimates demonstrates that large scale regional structures can be reasonably approximated along each profile as two dimensional with dominant geoelectric strikes of either 30 or 15 depending on profile location these profile specific strike directions are consistent with a local clockwise rotation of crustal structures in the southern intermontane and omineca belts suggested by others based on paleomagnetic data and palinspastic reconstructions comparing the resistivity models derived from two dimensional inversions of the distortion corrected data along each profile allows us to construct an orogen scale three dimensional resistivity model for southern british columbia generally the model shows a resistive upper crust overlying a conductive lower crust the resistivity of the lower crust beneath the intermontane belt is independent of latitude and is similar for all profiles in stark contrast a 2 orders of magnitude variation in lower crustal resistivity is observed along strike in the omineca belt with higher conductivities to the south in the region of eocene extension and lower conductivities to the north in the unextended part of the belt such spatial association has noteworthy implications for the cause of lower crustal resistivity in active or recently active young regions our preferred interpretation of the observed lower crustal resistivities with other geophysical data is in terms of fluids with brines dominating for the most part but partial melt possible at the base of the crust in specific localities we attribute the along strike variation in the omineca belt mostly to variation in fluid content and interconnectivity with the lowermost crust of the southern omineca belt being partially molten this physical state difference is a consequence of degree of extension and implies that mantle derived fluids are important for lower crustal resistivity

directional wavelet analysis on the sphere application to gravity and topography of the terrestrial planets 1 the spectral relations admittance and correlation between gravity and topography are often used to obtain information on the density structure flexural support and heat flow of planetary lithospheres mapping spatial variations in these quantities requires spatiospectral analysis techniques here we describe the application of a directional continuous spherical wavelet transform using a wavelet basis constructed from the superposition of azimuthally adjacent complex morlet wavelets in a manner similar to the fan wavelet developed in the plane the method is applied to gravity and topography of the earth venus mars and the moon the wavelet coefficients are used to compute isotropic and directional wavelet autospectra and cross spectra which are then combined to form the admittance and correlation functions the resulting maps offer insights into lithospheric structure of the terrestrial planets in particular we show that the earth and venus have uniformly low positive admittance and high correlation whereas mars and the moon display hemispherical contrasts with large negative and anisotropic coefficients coinciding with lowlands as has long been known the two largest impact basins in the inner solar system the south pole aitken basin on the moon and the hellas basin on mars display low positive admittance and high correlation indicating isostatic compensation in contrast most other impact basins particularly the martian and lunar mascons show negative coefficients at low wavelet degrees suggesting flexural support by a strong lithosphere these results imply that although simple isotropic flexural models can account for most observations future models may need to incorporate anisotropy as an additional parameter

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

systematics of variations in excess al and al ti in sediments from the central equatorial pacific the proposal that the accumulation rate ar of scavenged al in central equatorial pacific sediments is controlled by bulk particle flux and therefore can be used as a paleoproduction proxy is tested by examining the relationship between al and ti using data on core wec8803b gc51 published by murray et al 1993 the downcore profile of the excess al ar follows the bulk ar consistent with the al scavenging hypothesis and is due to a relatively constant abundance of excess al down core however there have been large changes in the aluminosilicate ar to this region suggesting that changes in the al ti value do not solely trace variability in the delivery rate of scavenged al assuming that the excess al in this core is not the result of an analytical artifact we present an alternative to the al scavenging hypothesis that involves authigenic clay mineral formation or some other process causing the occurrence of excess al at a relatively constant abundance of the sediment this process may well be independent of bulk particle flux or paleoproduction

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

isostatic response and anisotropy of the eastern himalayan tibetan plateau a reappraisal using multitaper spectral analysis 1 we estimate the variation in strength of the eastern himalayan tibetan lithosphere from 2d gravity and topography coherence functions calculated with the thomson slepian multitaper technique the coherence shows evidence of structural weakness of the plate in the n s direction aligned with the direction of maximum compression the tibetan plateau is characterized by an effective elastic thickness te that is much lower than would be expected from its large crustal thickness and the additional strength contributed by a coherently folded mantle inferred from shear wave splitting the seismogenic thickness indicates that only the upper crust is contributing to the elastic strength and not the mantle the predominant weakness of the lithosphere in the n s direction revealed from the anisotropy in the coherence function suggests that the mantle may not be directly coupled to crustal deformation beneath the tibetan plateau

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

distribution of major trace and rare earth elements in surface sediments of the wharton basin indian ocean bulk chemical analyses of eighteen surface sediments taken on an east west transect lat 15 40 s in the wharton basin indian ocean were analysed for major trace and rare earth elements ree by inductively coupled plasma atomic emission spectrometry icp aes the relationship between al and k suggests in situ formation of authigenic phillipsite the relationship between p and ca suggests accumulation of biogenic apatite fish bone debris from high biological productivity waters which is confirmed by the linear relationship with p and cu a strong positive correlation among redox sensitive metals mn fe cu ni and zn suggests that they are incorporated into a ferromanganese oxide phase probably micronodules calcareous oozes from the transect have low ree abundance la 8 ppm with a very strong negative cece value 0 41 siliceous oozes have high ree abundance la 28 ppm with moderate positive cece 1 24 and red clays have highest ree abundance la 66 ppm with a weak positive 1 05 to moderate negative cece 0 73 shale normalized ree patterns and layb n ratios record similar enrichment of heavy ree hree relative to light ree lree in all sediment types this might be due to retention of the bottom water ree pattern and the presence of fish bone debris correlation coefficient and r mode factor analyses suggest a very strong positive association r2 0 9 of ree with p and thus most likely with biogenic apatite fish bone debris as p is directly associated with biological activity it is thus related to surface primary productivity ree in the wharton basin sediments appear to reflect a combination of surface water effects and diagenetic processes

three dimensional flexural modelling of the ebro basin ne iberia summary the ebro basin the southern foreland basin of the pyrenees has undergone a complex evolution in which apart from the pyrenees the iberian range and the catalan coastal ranges have played an important role both as sediment sources and as basin confining structures the deflected basement underlying the ebro basin dips north suggesting a lithospheric scale control on the structure of this basin this is compatible with the results of subsidence analyses which show that the study area is not in a local mode of isostatic compensation in order better to understand the mechanisms that led to the present configuration of the ebro basin and particularly the relevance of the various kinds of un loading e g surrounding fold and thrust belts basin topography subsurface loads we carried out a 3 d kinematic modelling study that accounts for the flexural state of the lithosphere subjected to various loads applied at its lateral boundaries and the sedimentary fill of the basin we also included the effect of neogene extensional tectonics along the eastern basin margin which is related to the opening of the valencia trough we show the suitability of the 3 d lithospheric scale flexural modelling approach to the study of ne iberia modelling results point to a relatively strong lithosphere in this area with values of effective elastic thickness ranging from 10 to 35 km in the ebro basin increasing towards the pyrenees we also find that the topographic tectonic load itself is insufficient to explain the observed basement deflection thus an extra subsurface load beneath the pyrenees corresponding to the underthrusted iberian lithosphere is required the effect of lithospheric stretching in the valencia trough on the ebro basin is appreciable only in its eastern part where the lithosphere was uplifted this had considerable repercussions on the sedimentary and erosional regime of the ebro basin we have analysed the link between the stretching related tectonically uplifted areas and the erosional patterns observed onshore northeast iberia

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

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

chemistry of surface sediment along a north south transect across the equator in the central indian basin an assessment of biogenic and detrital influences on elemental burial on the seafloor nineteen surface sediment samples collected along a north south transect across the equator in the central indian basin cib were analysed for their major minor and rare earth element ree composition in absence of the element flux data we demonstrate here the association of various elements and their preferential sinks using inter elemental associations in sediment exhibiting latitudinal dissimilarity biogenic sedimentation north of the equator 1 to 4 n latitudes is dominated by carbonate ooze whereas south of the equator 4 to 14 s latitudes is dominated by siliceous ooze carbonate ooze deposition 60 caco3 above 4400 m and its drastic reduction to 80 of their respective total content in all the three sediment zones these observations suggest a close link between ba and a mn oxide phase on the other hand nearly overlapping latitudinal trends of alxs with carbonate in areas north of the equator and opal south of the equator strong positive correlation of alxs with opal r 0 8 similar ratios between carbonate free opal and alxs 16 in both biogenic sediments suggest a close link between productivity and alxs the alxs proportion in both biogenic sediment types is around 28 of the total al suggesting nearly the same level of primary productivity along the transect 4 n to 14 s which supports the estimates of berger berger w h 1989 global maps of the ocean productivity in w h berger v s smetacek g wefer eds productivity of the ocean present and past dahlem workshop reports wiley chichester pp 429 455 and kabanova kabanova j g 1968 primary production of the northern part of the indian ocean oceanology 8 214 255 i e 70 g cm 2 y 1 the rees exhibit mutual coherence in the sediment r 0 7 however they display an increasing affinity towards mn r increases from 0 6 to 0 8 and decreasing affinity towards ti r decreases from 0 8 to 0 3 with increasing atomic number such a subtle but systematic variation within ree group suggests preferential burial of heavy ree by mn oxide phase and light ree by detrital phase

integrated teleseismic studies of the southern alberta upper mantle this paper presents results from a teleseismic experiment conducted across the hearne province in south central alberta data from an array of nine portable broad band seismographs deployed along a 500 km nw se array have been supplemented with recordings from two canadian national seismograph network stations p wave delay times from 293 earthquakes have been inverted for upper mantle velocity structure below the array the recovered model reveals high velocities beneath much of the southern hearne province to depths of 200 250 km which are interpreted as deep seated lithospheric structure contrary to recent tectonic models these results suggest that the hearne lithosphere has remained intact in particular it appears unlikely that evidence for extensive lower crustal melting derives from lithospheric delamination however the results admit the possibility that high mantle conductivity as revealed in magnetotelluric studies originates through small volumes of connected hydrous minerals or other c

structure and fabric of the crust and uppermost mantle in the northern canadian cordillera from rayleigh wave tomography abstract the seismic structure and fabric of the lithosphere and underlying mantle beneath the northern canadian cordillera provides important constraints on its evolution and current tectonics however it is poorly characterized due to historically sparse networks of seismic instruments we use data from past and recently deployed networks of broadband seismic stations in northwestern canada and measure rayleigh waves propagating between all available pairs of seismic stations using two complementary techniques ambient noise and teleseismic two station interferometry the rayleigh wave data are processed to obtain phase velocity dispersion curves that are inverted for phase velocity maps at periods between 8 and 80 s to first order these maps show high velocity anomalies within the canadian shield and low velocity anomalies within the cordillera at all periods at short periods 30 s mostly sensitive to uppermost mantle depths high velocity anomalies of the canadian shield extend west past the cordilleran deformation front and suggest the presence of cratonic lithosphere beneath the cordillera whereas the lowest velocities underlie the allochtonous terranes anisotropy within the crust and uppermost mantle exhibits fast axis orientations aligned with the major faults and fabric of the cordillera and show evidence for vertical changes in anisotropy these results provide new constraints on geodynamic models proposed to explain neotectonic deformation in this area

multiscale dynamics of the tonga kermadec subduction zone our understanding of mantle convection and the motion of plates depends intimately on our understanding of the viscosity structure of the mantle while geoid and gravity observations have provided fundamental constraints on the radial viscosity structure of the mantle the influence of short wavelength variations in viscosity is still poorly understood we present 2 d and 3 d finite element models of mantle flow including strong lateral viscosity variations and local sources of buoyancy owing to both thermal and compositional effects we first use generic 2 d models of a subduction zone to investigate how different observations depend on various aspects of the viscosity structure in particular the slab and lower mantle viscosity and the presence of a low viscosity region in the mantle wedge above the slab we find that 1 the strain rate provides a strong constraint on the absolute viscosity of the slab 10 23 pa s 2 stress orientation within the slab is sensitive to the relative viscosity of the slab lower mantle and the wedge and 3 the stress state and topography of the overriding plate depend on the wedge viscosity and local sources of buoyancy in particular the state of stress in the overriding plate changes from compression to extension with the addition of a low viscosity wedge we then use observations of strain rate stress orientation dynamic topography and the geoid for the tonga kermadec subduction zone as simultaneous constraints on the viscosity and buoyancy in a 3 d regional dynamic model together these observations are used to develop a self consistent model of the viscosity and buoyancy by taking advantage of the sensitivity of each observation to different aspects of the dynamics over a broad range of length scales the presence of a low viscosity wedge makes it possible to match observations of shallow dynamic topography and horizontal extension within the backarc and down dip compression in the shallow portion of the slab these results suggest that a low viscosity wedge plays an important role in controlling the presence of backarc spreading however for a model with a low viscosity and low density region that provides a good fit to the observed topography we find that a reduction of the slab density by a factor of 1 3 relative to the reference density model is required to match the observed geoid these results suggest that compensation of the slab by dynamic topography may be a much smaller effect at short to intermediate wavelengths than predicted by long wavelength modelling of the geoid

continental margin deformation along the andean subduction zone thermo mechanical models the chilean andes extend north south for about 3000 km over the subducting nazca plate and show evidence of local rheological controls on first order tectonic features here rheological parameters are tested with numerical models of a subduction zone incorporating slab pull and upper plate convergence which calculate the development of stress and strain over a typical period of 4 myr the models test the effects of subduction interface strength arc and fore arc crust rheology and arc temperature on the development of superficial near surface faulting as well as viscous shear zones in the mantle deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities upper plate shortening and trench advance are both correlated and favored to a first order by upper plate weakness and to a second order by interface strength in cases of a strong interface a weak forearc crust is dragged downward by tectonic erosion a scenario for which indications are found along the northern chilean margin in contrast for a resistant forearc the slab pull force transmits to the surface and produces topographic subsidence this process may explain present day subsidence of the salar de atacama basin and or the persistence of a central depression specific conditions for northern chile produce a shear zone that propagates from the subduction zone in the mantle through the altiplano lower crust into the subandean crust as proposed by previous studies models with a weak interface in turn allow buoyant subducted material to rise into the continental arc in case of cessation of the slab pull this buoyant material may rise enough to change the stress state in the continental crust and lead to back arc opening in a case of young and hydrated oceanic plate forced by the slab pull to subduct under a resistant continent this plate is deviated and indented by the continental mantle and stretches horizontally at 100 km depth this situation might explain the flat wadati benioff zone of central chile

a regional study of shear wave splitting above the cascadia subduction zone margin parallel crustal stress recordings of local earthquakes from 16 three component broadband seismic stations in southwestern british columbia washington and northern oregon are used to study regional variations of shear wave anisotropy in the north american plate above the subducting juan de fuca plate there is evidence for shear wave splitting at all sites with good agreement of fast polarization directions and travel time delays at adjacent stations most stations exhibit fast directions parallel to the strike of the margin with anisotropy of 1 2 these fast polarization directions are consistent with earthquake focal mechanisms and borehole stress studies indicating that the observed anisotropy is likely due to crustal stresses i e extensive dilatancy anisotropy theory the margin parallel stresses may be due to oblique subduction of the juan de fuca plate however at the station closest to the coast ozb the fast direction shows a more margin normal orientation that may be associated with the proximity of the locked portion of the underlying subduction thrust fault

tectonic fabric of the subcontinental lithosphere evidence from seismic magnetotelluric and mechanical anisotropy abstract knowledge of anisotropy is key to our understanding of tectonic fabrics in the lithosphere and sublithospheric mantle anisotropy provides a unique constraint on the character of past and present deformation and thus is critical to our understanding of dynamics of the plate tectonic system in particular how continents with their thick lithospheric mantle roots form stabilize and interact with underlying mantle regions improved observational methods and rapid developments of geophysical inversion techniques are contributing to more realistic earth models that include various types of anisotropy and heterogeneity a more robust characterization of anisotropic parameters can be achieved by integrating complementary datasets incorporating constraints from seismology magnetotellurics geodynamics and mineral physics valid comparisons between different measures of anisotropy however require a solid understanding of the principles and limitations of the various techniques to this end we briefly summarize a new compilation of original articles with a common focus on anisotropy of continental lithosphere including theory laboratory experiments and observations that span a range of different geophysical techniques

density and lithospheric thickness of the tharsis province from mex mars and mro gravity data 1 radio science tracking of mars express mars experiment and mars reconnaissance orbiter produced high resolution martian gravity data applying localized spectral analysis on the new gravity data sets we study the surface density and lithospheric elastic thickness in the tharsis province the gravity signal is predicted with geophysical models either including bottom loading top bottom model or taking into account the loading history top top model volcanic shields are mainly composed of high density lava but their construction could have begun with lower density lava with the exception of ascraeus mons which has a top of lower density buoyant bottom loading may have been present in the form of a mantle plume under olympus mons the elastic thickness was much larger at olympus mons than at other volcanoes suggesting large spatial variations of heat flux during the hesperian alternatively small elastic thicknesses could be artifacts reflecting the presence of very localized high density crustal intrusions beneath the volcanoes thaumasia highlands were probably supported by a mantle plume at the time of formation in valles marineris top bottom models predict low densities and large elastic thicknesses in conflict with the basaltic rock composition and hesperian age of the valley dense mafic dikes underlie the western part of the valley the top top model serves to test another scenario in which the trough is formed with sedimentary infilling removed much later by erosion the elastic thickness increasing in between at the large volcanoes the relation between gravity and topography is anisotropic probably because of density variations

lithospheric anisotropy structure inferred from collocated teleseismic and magnetotelluric observations great slave lake shear zone northern canada 1 accurate interpretation of sks shear wave splitting observations requires inherently indeterminate depth information magnetotelluric electrical anisotropies are depth constrained and thereby offer possible resolution of the sks conundrum mt and teleseismic instruments deployed across the great slave lake shear zone northern canada investigated lithospheric anisotropy and tested a hypothesis that seismic and electrical anisotropy obliquity can infer mantle strain shear sense lithospheric mantle mt strike n60 e differs significantly from crustal mt strike n30 e sks splitting vectors outside the shear zone exhibit single layer anisotropy with fast axis parallel to upper mantle mt strike and oblique to present day plate motion n135 w back azimuth sensitivity at sites within the 30 km wide shear zone imply more complex layering with two layer inversion yielding an upper layer of n20 e and a lower layer of n66 e the mt data help to constrain the depth location of sks anisotropy and taken together support a model of fossil lithospheric anisotropy

effective elastic thickness of the arabian plate weak shield versus strong platform the fan wavelet method has been employed to calculate high resolution maps of variations of the effective elastic thickness eet for the arabian plate and surroundings as the initial data we use high resolution gravity field topography and recent models of sedimentary basins the western part of the plate is generally characterized by low to midvalues of eet 10 30 km while the eastern one by high values 50 km and more in the core this finding confirms that the pronounced asymmetry of the plate is rather associated with fundamental structural differences of the lithosphere than with a forced tilt of the plate due to the rifting in the west southwest and subduction in the northeast therefore the high topography in the western part of the plate is likely supported by relatively hot mantle that is also responsible for the decrease of eet these results are generally in agreement with recent seismic tomography models

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

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

seismic and mechanical anisotropy and the past and present deformation of the australian lithosphere abstract we interpret the three dimensional seismic wave speed structure of the australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation we infer the fossil strain field from the orientation of gravity anomalies relative to topography bypassing the need to extrapolate crustal measures and derive the current direction of mantle deformation from present day plate motion our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation the distribution of azimuthal seismic anisotropy is determined from multi mode surface wave propagation mechanical anisotropy or the directional variation of isostatic compensation is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands the joint interpretation of seismic and tectonic data resolves a rheological transition in the australian upper mantle at depths shallower than 150 200 km strong seismic anisotropy forms complex patterns in this regime the seismic fast axes are at large angles to the directions of principal shortening defining a mechanically coupled crust mantle lid deformed by orogenic processes dominated by transpression here seismic anisotropy may be considered frozen which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile the azimuthal seismic anisotropy below 200 km is weaker and preferentially aligned with the direction of the rapid motion of the indo australian plate the alignment of the fast axes with the direction of present day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle motion expressed in the hot spot reference frame matches the seismic observations better than the no net rotation reference frame thus seismic anisotropy supports the notion that the hot spot reference frame is the most physically reasonable independently from plate motion models seismic anisotropy can be used to derive a best fitting direction of overall mantle shear

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

variations of the lithospheric strength and elastic thickness in north america we evaluate the effect of temperature variations on strength and effective elastic thickness te of the lithosphere of the north american na continent to this purpose we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle these thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models na07 and sl2013sv the crustal rheology was defined using nacr14 the most recent na crustal model this model specifies seismic velocities and thickness for a three layer model of the crystalline crust strength in the lithosphere and in the crust has similar distributions indicating that local geotherms play a dominant role in determining strength rather than crustal composition a pronounced contrast is present in strength between cratonic and off cratonic regions lithospheric strength in the off cratonic regions is prevalently localized within the crust and te shows low values 150 km in contrast to previous results our models indicate that phanerozoic regions located close to the edge of the cratons as the appalachians are characterized by low strength we also find that locally weak zones exist within the cratons e g beneath the intracratonic illinois basin and midcontinent rift seismic tomography models na07 and sl2013sv differ mainly in some peripheral parts of the cratons as the proterozoic canadian platform the grenville and the western part of the yavapai mazatzal province where the integrated strength for the model na07 is 10 times larger than in model sl2013sv due to a temperature difference 200 c in the uppermost mantle the differences in te between the two models are less pronounced in both models proterozoic regions reactivated by meso cenozoic tectonics e g rocky mountains and the mississippi embayment are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer intraplate earthquakes are distributed along the edges of the cratons demonstrating that tectonic stress accumulates there while the cores of the cratons remain undeformed in both models intraplate earthquakes occur in weak lithosphere 0 5 1013 pa s te 15 km or near the edges of strong cratonic blocks characterized by pronounced contrasts of strength and te

lith5 0 a revised crustal model for canada based on lithoprobe results summary during the last 15 yr numerous seismic refraction experiments have been conducted overthe canadian landmass most as part of the lithoprobe initiative the new results providethe basis for revising existing global models of crustal structure e g crust5 1 mooney et al 1998 which do not incorporate the new lithoprobe data the crustal model discussedhere lith5 0 combines the lithoprobe and crust5 1 data to provide better constraints oncrustal structure in canada the objectives of the study are to 1 compile available new datainto a single consistent database of crustal thickness and velocity including the uppermostmantle pn velocity and 2 to construct a revised 5 5 crustal model of the canadianlandmass in a format that is generally compatible with existing global models the lithoproberesults suggest a 5 5 km average root mean square rms crustal thickness difference betweenlith5 0andcrust5 1 evenwiththeincorporationofthenewdata thegeographiccoveragein north and central canada remains sparse the new model does not explicitly incorporategeological constraints but instead relies on spherical spline interpolation for extrapolatingvalues into regions that lack control to illustrate the implications of this new crustal model we use lith5 0 to correct for the isostatically compensated crustal heterogeneity in order toobtain improved estimates of the dynamic surface topography the latter provides importantconstraints on the density structure of the uppermost mantle