a seismic study of lithospheric flexure in the vicinity of tenerife canary islands seismic data have been used to determine the crustal and upper mantle structure of tenerife canary islands a volcanic island of tertiary age located on 140 ma oceanic crust reflection data show that oceanic basement dips gently towards the island forming a flexural moat which is infilled by 2 3 km of well stratified material the moat is characterised by a major angular unconformity which we attribute to volcanic loading of pre existing oceanic crust and overlying sediments and the subsequent infilling of the flexure by material that was derived at least in part from the islands refraction data show that the flexed oceanic crust has a mean thickness of 6 41 0 42 km and upper and lower crustal velocities of 4 8 5 4 km s 1 and 6 7 7 3 km s 1 respectively the flexure which has been verified by gravity modelling can be explained by a model in which tenerife and adjacent islands have loaded a lithosphere with a long term 106 yr elastic thickness of approximately 20 km seismic and gravity data suggest that up to 1 5 105 km3 of magmatic material has been added to the surface of the flexed oceanic crust which assuming an age of 6 16 ma for the shield building stage on tenerife implies a magma generation rate of about 0 006 to 0 02 km3 a 1 this rate is similar to estimates from other african oceanic islands e g reunion and cape verdes but is significantly less than that which has been calculated at hawaii there is no evidence in either the seismic or gravity data that any significant amount of magmatic material has underplated the flexed oceanic crust the crustal and upper mantle structure at tenerife therefore differs from other oceanic islands such as hawaii and marquesas where 4 km of underplated material have been reported

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

on the accuracy of digital bathymetric data the global 5 arcmin gridded topography data etopo 5 are based on contour maps rather than original soundings and have large errors the artificial statistical distribution generated by digitizing contours makes these data unsuitable for use in regression models for depth age variations their amplitude spectrum is bounded by a frequency 4 power law so that they should not be used when the gravity topography transfer function is important such as in studies of flexural isostasy and mantle convection i assess the accuracy of 14 491 069 digital ship soundings in 2253 cruise surveys collected between 1955 and 1992 in the lamont doherty earth observatory on line data base by analyzing 329 058 crossover errors coes at intersecting ship tracks five percent of cruises with internal coes yield root mean square coe amplitudes exceeding 500 m all of these have errors in digitizing two way travel time from analog precision depth recorder traces twenty eight cruises were found which had errors caused by misinterpretation of the nominal sound velocity used when travel times were reported as nominal depths two nominal sound velocities in common use differ by 2 5 an amount which is often undetectable producing uncertainties in depth of this magnitude ship data have been acquired at different rates over time with the peak of activity in the early 1970s although present technologies can yield very accurate data these are acquired at a rate which is small with respect to the total available data the cumulative median global coe has remained constant at 26 m since the late 1970s most recent data acquisition has been in the northern hemisphere oceans and the oldest and least accurate data are in the southern oceans where median coes are 100 250 m the majority of the data in the south pacific were acquired before the advent of satellite navigation

thermal and mechanical properties of the cape verde rise the cape verde rise represents an extreme case of the process of oceanic midplate volcanism the regional depth and geoid anomalies relative to the plate model are the largest observed peaking at 2 4 km and 12m respectively the late tertiary volcanoes rest on mesozoic aged seafloor indicating that the lithosphere was about 130 m y old when loaded depth and geoid anomalies in the cape verde region are examined using both forward filtering and admittance techniques to determine the thermomechanical properties of the swell the effective elastic thickness for the plate supporting the volcanoes is 28 4 km and the compensation depth for the broad swell is 69 10 km these two results combined with heat flow observations from the rise are consistent with a model for reheating of the lithosphere beneath cape verde such that the average thermal gradient is increased by 35 while there is no doubt that the cape verde lithosphere has been reheated it is unlikely that the entire depth anomaly is explained by thermal anomalies confined to the lithosphere a linear programming inversion of the geophysical data places an upper bound of 1 7 km on the portion of the depth anomaly due to lithospheric reheating there are no permissible thermal models fitting all of the geophysical data that do not require either dynamic support from or elevated temperature within the asthenosphere

a note on the correction of ocean floor depths for sediment loading the measured physical properties of sediments recovered in a number of deep sea drill holes in the north atlantic have been used to derive expressions for the correction to basement depths required to account for loading by sediments the average density and porosity over a given thickness of sediment show an approximate linear dependence on sediment thickness this behavior can be explained as primarily the result of compaction if the compaction of the sediments is neglected in correcting basement depths for sediment loading errors of a few hundred meters in the corrected depths can be made a sediment loading correction based on the observed variation of density with depth can give corrected depths to better than 100 m with remaining uncertainties due to variablity in sediment properties and rebound effects on the measured densities a relation between sediment thickness and vertical travel time was obtained from sonic velocity measurements on samples from drill holes sonobuoy wide angle reflection experiments and direct correlation of seismic reflectors with downhole lithology the sediment loading correction can then be expressed directly in terms of the vertical travel time measured along seismic reflection profiles the loading correction obtained here for the north atlantic may not be applicablemore in areas with different sedimentary enviroments when direct measurements of physical properties in drill holes are not available in such areas it is shown that it is preferable to assume a compaction curve for the appropriate lithology rather than to use observed seismic velocities together with a velocity density relationship less

seasat observations of lithospheric flexure seaward of trenches lithospheric flexure seaward of deep ocean trenches is evident in seasat altimeter observations of the marine geoid in fact mechanical models of lithospheric flexure can be tested directly on the seasat altimeter data we have used a simple elastic model for the oceanic lithosphere and after least squares adjustments have recovered estimates of model parameters including outer rise or amplitude or wavelength and effective lithospheric thickness effective lithospheric thicknesses have been recovered for six regions the mariana the kuril the philippine the aleutian the izu bonin and the middle america or s these results support the proposition that effective thickness te increases with age of lithosphere in approximate accord with the relation te c age1 2 where c 4 km m y 1 2 in fact our altimetric results agree more closely with this relation than do published results based on bathymetric data the close agreement with the thickness age relation suggests that there is no longer any need to assume that significant horizontal compression acts across the kuril marianas and izu bonin trenches this thickness age relation implies that flexural strength of the oceanic lithosphere is temperature controlled

features of crustal structure under the canary islands abstract the data and interpretation of a seismic refraction project carried out in the canary islands in 1977 and 1979 are presented the moho is reached at about 15 km under fuerteventura gran canaria and tenerife whereas under lanzarote a value of 11 km has been found the velocity at the surface varies even within one island from 2 8 to 4 4 km s there are variations in the velocity of the main crustal layer under the volcanic deposits a continuous velocity depth function seems to be more appropriate for the island of tenerife contrary to fuerteventura lanzarote and gran canaria which seem to have a layered structure there is evidence for pronounced structural differences between lanzarote and fuerteventura the mantle velocity has a value of about 8 0 km s under the western islands whereas a very low value of 7 4 km s has been found under lanzarote and fuerteventura despite the clear structural differences between the islands our results leave little doubt about the oceanic character of all the islands the differences between the eastern and western group may be due to the islands being situated close to the african continental margin the seismic velocities under the volcanic deposits do not support the idea of a common basement for all the islands they rather suggest independent volcanic edifices finally a comparison of the structure of the crust under the canary islands with that under other volcanic islands reveals remarkable similarities

variation of the apparent compensation depth of hotspot swells with age of plate abstract most oceanic hotspot swells are associated with anomalously shallow seafloor and broad geoid anomalies these depth and geoid anomalies are generally positively correlated and linearly related the linear relationship can be used to infer an apparent depth of isostatic compensation of the swell we have analysed geoid and depth anomalies over 23 oceanic hotspot swells almost the majority of oceanic hotspots and deduced the apparent depth of compensations of these features the data indicate a linear increase of the compensation depth with the square root of seafloor age t on the order of 7 km m y 1 2 this trend is comparable to that predicted by conductive cooling models for the deepening of the isotherms inside the oceanic lithosphere

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

evidence from gravity and topography data for folding of tibet bouguer gravity and topography data from tibet suggest that the surface of the plateau and the subsurface density interfaces are warped into two series of ridges and troughs trending parallel to the collision zone with wavelengths of 150 and 500 km these folds are superimposed on an overall state of isostatic compensation owing to crustal thickening such folding is predicted from models of compressional shortening of a rheologically layered plate the results thus suggest the presence of a weak decoupling zone between the tibetan crust and upper mantle

the stratigraphy and structure of the faroese continental margin this paper presents a summary of the stratigraphy and structure of the faroese region as the faroese area is mostly covered by volcanic material the nature of the pre volcanic geology remains largely unproven seismic refraction data provide some indications of the distribution of crystalline basement which probably comprises archaean rocks with the overlying cover composed predominantly of upper mesozoic cretaceous and cenozoic strata the cenozoic succession is dominated by the syn break up faroe islands basalt group which crops out on the faroe islands where it is up to 6 6 km thick and shelf areas post break up sediments are preserved in the adjacent deep water basins including the faroe shetland basin seismic interpretation of the post volcanic strata shows that almost every sub basin in the faroe shetland basin has been affected by structural inversion particularly during the miocene these effects are also observed on the faroe platform the munkagrunnur ridge and the fugloy ridge where interpretation of low gravity anomalies suggests a large scale fold pattern the structure of the iceland faroe ridge which borders the nw part of the faroe area remains ambiguous the generally thick crust together with the absence of well defined seawards dipping reflectors may indicate that much of it is underlain by continental material

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

effects of canary hotspot volcanism on structure of oceanic crust off morocco analysis of over 6400 km of multichannel seismics mcs and 50 sonobuoy reflection and refraction experiments reduced both in the domain of x t and tau p shows that a region within the jurassic quiet zone off morocco underwent dramatic changes as a result of the passage of the lithosphere over the canary hotspot commencing approximately 60 ma a seismic unit ucf interpreted as volcanic in origin is observed within the sediments in a region characterized by a broad bathymetric swell it shows diffractions from its upper surface and an internally chaotic seismic facies and pinches out between sedimentary units of continuous subparallel facies a velocity inversion is noted between the ucf 4 7km s and the underlying sediment 3 1 km s the ucf is time transgressive it lies near the cretaceous s tertiary boundary in the northern portion of the study area and is younger to the south kinematic studies of the movement of the canary hotspot relative to africa show that the hotspot first appeared off nw africa about 60 ma and was located beneath oceanic crust in the region where the ucf is observed depth to basement measurements in areas not effected by the hotspot show a consistent linear trend of increased depth with age in areas effected by the hotspot the thermal rejuvenation is evident as basement depths shoal with increased proximity to the present hotspot the reheating of the crust resets the thermal age of the lithosphere with many of the properties of crust of a younger age subsidence curves of the reheated crust off morocco show good correlation to subsidence curves of other reheated crust on a global basis a zone characterized by high crustal velocities 7 1 7 4 km s and greater crustal thicknesses by 1 2 km is observed in an area that corresponds to the bathymetric swell the region of the ucf and the reelevated basement the high velocities and increased crustal thickness are interpreted to be the result of underplating and assimilation of existing oceanic crust caused by the canary thermal anomaly the presence of high crustal velocities coupled with a thickened crustal section has been noted on various passive margins of the world they have generally been attributed to the thermal processes associated with continental rifting off morocco we believe that similar thermally induced phenomena have occurred but that here the heat anomaly was the midplate volcanism associated with the canary hotspot

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

observational constraints on models of the hawaiian hot spot swell a detailed analysis of the topography and geoid associated with the hawaii emperor seamount chain reveals that while geoid topography ratios are high over the southeastern part of the chain they fall off with distance away from the current hot spot location the topographic expression of the swell has two maxima near the intersections between the seamount chain and the murray and molokai fracture zones respectively both geoid and topography rapidly decrease westward of the intersection with the murray fracture zone it appears that the region of the pacific plate between the two fracture zones has been more susceptible to reheating and hot spot penetration than elsewhere along the chain alternatively the observed pattern may be indicative of time variation in the strength of the hawaiian hot spot

the canary islands swell a coherence analysis of bathymetry and gravity summary the canary archipelago is an intraplate volcanic chain located near the west african continental margin emplaced on old oceanic lithosphere of jurassic age with an extended volcanic activity since middle miocene the adjacent seafloor does not show the broad oceanic swell usually observed in hotspot generated oceanic islands however the observation of a noticeable depth anomaly in the basement west of the canaries might indicate that the swell is masked by a thick sedimentary cover and the influence of the canarian volcanism we use a spectral approach based on coherence analysis to determine the swell and its compensation mechanism the coherence between gravity and topography indicates that the swell is caused by a subsurface load correlated with the surface volcanic load the residual gravity geoid anomaly indicates that the subsurface load extends 600 km ssw and 800 km n and nne of the islands we used computed depth anomalies from available deep seismic profiles to constrain the extent and amplitude of the basement uplift caused by a relatively low density anomaly within the lithospheric mantle and coherence analysis to constrain the elastic thickness of the lithosphere t e and the compensation depth of the swell depth anomalies and coherence are well simulated with t e 28 36 km compensation depth of 40 65 km and a negative density contrast within the lithosphere of 33 kg m 3 the density contrast corresponds to a temperature increment of 325 c which we interpret to be partially maintained by a low viscosity convective layer in the lowermost lithosphere and which probably involves the shallower parts of the asthenosphere this interpretation does not require a significant rejuvenation of the mechanical properties of the lithosphere

depth and geoid anomalies over oceanic hotspot swells a global survey the broad depth and geoid anomalies associated with 23 hotspot swells in oceanic areas have been analyzed maximum height and geographical extent of the topographic swell and of the geoid anomaly as well have been measured for each hotspot the results indicate a clear increase of the topographic swell height with age of the underlying lithosphere from values in the range 300 500 m at young ages to values in the range 1 500 2 000 m at ages larger than 100 ma the geoid anomaly amplitude also increases with plate age from nearly zero close to mid ocean ridges to 6 8 m over old plates on the other hand the geographical extent of the swell does not show any clear relationship with plate age the mean lateral extent of swells range from 1 000 to 1 500 km swells located close to spreading ridges show a significant non zero depth anomaly but are associated with negligible geoid signal these results complete those of a previous study where the apparent compensation depth of oceanic hotspot swells was showed to increase linearly with the square root of plate age and coincide roughly with the base of the thermal lithosphere this trend may either be interpreted in terms ofmore lithospheric thinning or dynamical support besides both seem necessary to explain the observed bathymetry in proportion evolving with aging of the lithosphere less

constraints on thermal and mechanical structure of the oceanic lithosphere at the bermuda rise from geoid height and depth anomalies abstract the bermuda rise is a broad area of anomalously shallow seafloor presumed to be of thermal origin dating to the middle eocene this and other midplate swells have been modeled both by elevated temperature in a convecting layer at the base of the plate and as thermal expansion within the conducting portion of the lithosphere using recently developed techniques for the estimation of lithospheric flexure and temperature structure from geoid bathymetry and heat flow data we place constraints on the depth of the thermal load and limit the possible thermal mechanisms causing this swell our data include depth anomalies calculated from dbdb5c bathymetry data geoid anomalies using data from both seasat and geos 3 altimeter missions and published heat flow values the first technique we employ to determine the thermo mechanical properties of the swell is a forward filtering technique with which we determine flexural rigidity and compensation depth by successive fits to topography filtered to separate subsurface and surface loading the second method is an admittance technique with which we directly determine the geoid to topography ratios corresponding to the volcano and the swell both linear filtering and admittance approaches yield similar conclusions concerning the elastic plate thickness and compensation depth of the bermuda rise indicating that the effective elastic thickness for the plate supporting the volcanoes is 30 5 km and the compensation depth for the swell is 55 10 km finally we use the linear programming technique to obtain extremal bounds on temperature as a function of depth and time in the lithosphere we find that the depth and geoid anomalies heat flow and elastic thickness observations are inconsistent with a mechanism for formation of the bermuda rise which consists solely of thermal anomalies confined to depths greater than 50 km at the initiation of hotspot activity if we use thermal rejuvenation alone to model the bermuda rise the observations require that it be accompanied by substantial reheating to shallow depth in the lithosphere on the basis of the data presented in this paper we cannot distinguish between a reheating model and a convection model for the origin of the bermuda rise though based on our geothermal modeling we preclude convective support models that do not change temperatures within the thermal plate

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

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