northwest pacific tectonic evolution in the middle mesozoic abstract two magnetic bights implying pacific izanagi farallon p i f ridge ridge ridge rrr triple junctions occur in the northwest pacific basin one is formed in anomalies m26 m21 southeast of shatsky plateau while the second is formed in anomalies m10 cl1 the latter a new anomaly between m1 and m0 northeast of shatsky plateau the offset between these two bights is in a n40 e direction parallel to the p f relative motion during m20 to m10n time half spreading rates and directions and the assumption of symmetric orthogonal spreading permit the solution of the p i f velocity triangle and reveal an isosceles triangle with the i f rate equal to the p f rate this is a necessary condition for a stable fault fault ridge ffr triple junction which would migrate in the n40 e direction of the offset between magnetic bights the growth of the p f transform fault explains the increasing offset across the ends of anomalies m20 m10n projected onto the trace of shatsky plateau subsequent evolution led to the formation of a system of long transform faults offset by short spreading centers which we call the shatsky fracture zones and which extend eastward across the pacific from the plateau the original trace of the transform fault fracture zone that formed at the triple junction underlies the main mass of the shatsky plateau and would suggest that the formation of the plateau is at least coincidental with the existence of the ffr triple junction at 21 ma

the distribution of intraplate volcanism in the pacific ocean basin a spectral approach summary although the major linear island chains in the pacific ocean basin have been extensively studied relatively little attention has been given to the origin of the numerous seamounts and oceanic islands that are concentrated in the western pacific spectral analysis of shipboard gravity and bathymetry data provides a means for statistically estimating the distribution of this volcanism in space and time estimates of the power spectrum of the bathymetry fb and the average gravitational admittance z cross spectrum correlation of gravity and bathymetry were obtained as functions of age of the oceanic lithosphere by dividing the pacific plate into eight age regions a h the power spectrum of the bathymetry is indicative of the overall bathymetric roughness primarily volcanic in each region this roughness shows three gradations regions a d 140myr with many large volcanic features are very rough the average gravitational admittance is indicative of the degree of isostatic compensation of bathymetric features in each region and can be used to estimate the proportionh of total bathymetric power that was emplaced on relatively old lithosphere and the proportion that formed on young lithosphere at or near a mid ocean ridge crest the highest proportion of bathymetry that formed on old lithosphere occurs in region h fi 0 90 and the lowest proportion occurs in regions g and e fi 0 40 0 60 the power spectrum of the bathymetry and the gravitational admittance allow an estimate of the total bathymetric power pi ipb accounted for by intraplate volcanism intraplate power and the power pr 1 fi pb which is of on ridge origin ridge power at long wavelengths in regions e g the amount of on ridge volcanism exceeds the amount of intraplate volcanism pr pi indicating the presence of several large plateaus and rises which formed at ridge crests this suggests the

seamount topography by least squares inversion of altimetric geoid heights and shipborne profiles of bathymetry and or gravity anomalies summary altimetric measurements of sea surface heights have long been used to locate uncharted marine features newly available data sets geosat gm ers 1 and topex poseidon now allow the topography of submarine volcanoes to be accurately restored and a method based upon inverse modelling is presented in this paper this method allows a joint assimilation of altimetric data and shipborne profiles of bathymetry and or gravity anomalies when available furthermore uncertainties can be computed together with the topography which constitutes a noticeable improvement since these parameters are still missing in the usual bathymetric data bases tests are performed using simulated data in order to characterize the errors which may or must occur with the modelled topography of a previously unknown volcano in particular the errors due to data uncertainties and the lack of precise knowledge of sensitive parameters that are used in the modelling these errors can be maintained under the 100 m level rms for altimetric data with a measurement noise of 5 cm rms or less which is achieved for geosat ers 1 and topex poseidon altimeters an example of inversion of seasat data is presented in the case of a seamount in french polynesia

three dimensional estimation of elastic thickness under the louisville ridge a three dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry this technique is applied to the louisville ridge system to study the tectonic history of the region the inversion is performed as both a first order approximation and a nonlinear relationship between gravity and topography based on parker s 1973 equation while the higher order effect on the gravity anomaly is nearly zero for most of the region the magnitude is significant over the summits of the ridge nevertheless the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region lowering the value by 1 2 km compared with the linear result the incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly resulting in false sidelobe structure of nearly 20 mgal for large seamounts our elastic thickness estimates are compared with the contradictory values obtained in previous studies by cazenave and dominh 1984 and watts et al 1988 we find an increasing elastic thickness along the chain from southeast to northwest with a discontinuity along the wishbone scarp the jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge

on geoid heights and flexure of the lithosphere at seamounts the sea surface height has now been mapped to an accuracy of better than 1 m by using radar altimeters on board orbiting satellites the major influence on the mean sea surface height is the marine geoid which is an equipotential surface we have carried out preliminary studies of how oceanic volcanoes which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges contribute to the marine geoid simple one and two dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum previous studies based on gravity and bathymetry data and uplift subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thickness te increase with the age of the lithosphere at the time of loading the models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid ocean ridge crest will be associated with relatively low amplitude geoid anomalies about 0 4 0 5 m km of height while seamounts formed on relatively old lithosphere on ridge flanks will be associated with much higher amplitude anomalies 1 4 1 5 m km studies of the seasat altimetric geoid prepared by nasa s jet propulsion laboratory support these model predictions geoid amplitudes are relatively low over the mid pacific mountains and line islands which formed on or near a mid ocean ridge crest and relatively high over the magellan seamounts and wake guyots which formed off ridge direct modeling of the altimetric geoid over these features is complicated however by the wide spacing of the satellite tracks which can exceed 100 km and poor bathymetric control beneath individual satellite tracks in regions where multibeam bathymetric surveys are available models can be constructed that fit the altimetric geoid to better than 1 m studies of geoid anomalies over the emperor seamount chain for example suggest that these seamounts formed on 20 30 ma old oceanic lithosphere while anomalies over the louisville ridge and the hawaiian ridge suggest that these seamounts formed on lithosphere older than about 80 ma this indicates that geoid anomalies over bathymetrie features depend not only on the crustal structure prior to loading and the overall shape and density of the features but also on their tectonic setting we examine the implications of these results to studies which attempt 1 to separate temporal variations of the sea surface due to oceanographic effects 2 to predict seamounts directly from satellite derived gravity and geoid data and 3 to isolate the geoid effects of deep processes occurring beneath the lithosphere such as those due to mantle convection

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

lithospheric response to volcanic loading in the southern cook islands abstract the response of the ocean lithosphere to volcanic island loading has been investigated for the southern cook islands of rarotonga mangaia and the aitutaki mauke group using 1 estimates of the change with distance from the load of uplift of islands located on the flexure created arch 2 geoid height perturbations as observed with the geos 3 and seasat radar altimeters and 3 bathymetric evidence for the moat and arch about the island loads the effective flexural rigidity for rarotonga is found to be higher than for the older and nearby islands of mangaia and aitutaki this together with an earlier result for tahiti 1 suggests that stress relaxation of the lithosphere has occurred and that this layer behaves as a viscoelastic material with an effective viscosity of about 5 10 10 25 poise the observations for the flexural rigidity do not indicate any dependence on the age of the lithosphere at the time the loading occurred this suggests that there are no significant differences in lithospheric properties for plates in the age range of 6 7 10 7 years altimeter passes to the east of mauke indicate that there may be an uncharted submarine extension of the aitutaki mauke chain

the determination of geoid undulations and gravity anomalies from seasat altimeter data the global seasat altimeter data set has been edited and adjusted using a crossing arc procedure fixing one long arc to provide control in doing this a set of 168 master arcs were formed from repeat and near repeat arcs the adjustment carried out by d rowlands was done in a global net and four regional areas the average crossover discrepancy before the adjustment was 1 5 m while after the adjustment solving for a bias and trend it was 28 cm the examination of the crossover discrepancies by area indicated larger than normal discrepancies in certain areas such as the hudson bay the mediterranean and the north and baltic seas some of these discrepancies are caused by invalid tide values applied to the altimeter data in these areas the adjusted seasat sea surface heights were used to predict point values at 1 1 intersections in 10 different geographic regions for comparison with corresponding geos 3 values a mean difference geos 3 minus seasat of 1 3 0 67 m was found the mean difference being caused by different ellipsoid parameters the adjusted data were also used to determine mean surface heights and gravity anomalies in 37 9051 1 and 1178 5 equal area blocks with some of these values being on land these estimations were done using least squares collocation using covariances functions scaled to each 5 region the average predicted standard deviation of the 1 1 anomalies was 5 1 mgal and 2 7 mgal for the 5 blocks seasat values were compared to the geos 3 values finding a difference of 7 8 mgal and 0 87 m for the 1 1 data and 2 2 mgal and 0 76 m for the 5 data comparisons with terrestrial data showed no significant difference between the geos 3 seasat implied anomalies although a number of significant discrepancies have been resolved with the seasat data

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

topography and geoid undulations caused by small scale convection beneath continental lithosphere of variable elastic thickness summary the effect of subcontinental upper mantle convection on topography gravity and geoid undulation is studied with special regard to the effective elastic thickness of the continental lithosphere hel we designed a numerical model of the upper mantle lithosphere system consisting of a constant viscosity fluid overlayed by a partly elastic plate the boundary tractions stresses at the top of the convecting mantle model were applied to the lithosphere model for which we used the thin plate approximation the total thickness of the lithosphere was kept constant to 150 km while the elastic thickness was varied between 0 and 150 km the aspect ratio of the convecting mantle was varied between 0 7 and 3 and the rayleigh number between 104 and 106 our calculations demonstrate the effect of a thick elastic lithosphere on the surface observables peak to peak averaged amplitude of topography geoid undulation and gravity are strongly reduced and their spectra are low pass filtered for an aspect ratio less than 1 and a very thick elastic lithosphere topography tends to zero and while geoid undulation and gravity may even become negative above the up welling plume the spectral relation of geoid or gravity to topography admittance is nearly unaffected for hie 70km for aie 70km admittance values for all wavelengths generally decrease with increasing elastic thickness of the plate and may become negative geoid admittance for the basic mode increases with larger aspect ratio while gravity admittance for hie 100 km slightly decreases associating higher rayleigh numbers of the convecting fluid with lower viscosity an increase in rayleigh number leads to considerable decrease of the surface observables however admittance values are only slightly affected and decrease approximately linearly with increasing rayleigh number comparison of our results with observations of the topography spectrum and the free air gravity admittance for north america suggests that large aspect ratio cells beneath continents are unlikely however convection cells of aspect ratio 1 or less underlying an elastic lithosphere of at least 100 km thickness is in agreement with data the rayleigh number should be as high as 106

link between gold provinces low salinity high temperature 200 degrees c reducing fluids are poorly represented in ore forming processes yet appear to account for 90 percent of the primary gold mined to date such fluids have been implicated in the formation of many archcan greenstone witwatersrand slate belt and epithermal gold deposits these low salinity auriferous fluids occur in variable time space lithological sequence and tectonic settings and are most easily explained by metamorphic devolatilization of mafic and or graywacke successions metamorphism of chlorite calcite albite quartz assemblages e g mafic or graywacke at the greenschist amphibolite facies boundary occurs around 480 degrees or 20 degrees c for 3 5 kbars and produces large volumes of low salinity h 2 o co 2 fluid similar in composition to those recorded in many gold deposits widespread pyrite in the above assemblage typically leads to elevated levels of dissolved sulfur such conditions are ideal for gold transport as a molecular au s complex and can lead to deposits rich in gold relative to silver and base metals deposition of gold occurs in the temperature range of 250 degrees to 400 degrees c and can be facilitated by lower temperature lower sulfur activity and changes in oxygen activity lower sulfur activity is most readily achieved by fluid interaction with fe rich host rocks with the formation of pyrite or pyrrhotite whereas lower oxygen activity is readily achieved by interaction with carbonaceous metasediments low salinity fluids produced by metamorphic devolatilization provide a common theme embracing the formation of many archcan greenstone slate belt witwatersrand and potentially epithermal gold deposits it involves the common features of high geothermal gradient low salinity fluid high au ag high au base metals and broad synchroneity with metamorphism it also accounts for the considerable variability in hosting structures detailed timing relations host rocks alteration assemblages and geologic age of different deposits interestingly the similarities are explained by deeper processes the differences by processes at or near the depositional sites

the seasat altimeter mean sea surface model gridding techniques are used to combine an 18 day set of seasat altimeter data and two precisely computed seasat ephemerides in order to arrive at global contour maps of the mean sea surface topography the altimeter data have an rms agreement of 111 cm with the ss3 mean sea surface computed by means of the pgs s3 ephemerides and of 70 cm with the ss4 mean surface derived from the pgs s4 ephemerides while comparisons with the gem 10b 1 x 1 deg gravimetric geoid have yielded rms differences of 2 8 m those with a global mean sea surface derived from geos 3 altimeter data show rms differences of 1 3 m and 1 1 m for the cases of the ss3 and ss4 surfaces respectively an ss4 mean sea surface topograph is featured among the study findings presented further improvements in the representation of mean sea surface topography are expected with the development of more accurate gravity models for orbit computation

bathymetric prediction from seasat altimeter data the linear response function technique is used to analyze two 1300 km tracks of seasat altimeter data and corresponding bathymetry in the musician seamounts region north of hawaii bathymetry and geoid height are highly correlated in the 50 to 300 km wavelength range a predictive filter is developed which can operate on seasat altimetry in poorly surveyed oceanic regions to indicate the presence of major bathymetrie anomalies modeling of the bathymetry geoid correlation in the musician region is attempted using the elastic plate model the flexural rigidity d of the plate is not well constrained by our data but appears to lie in the range 5 1021 n m 5 1022 n m at the time of loading since the musician seamounts and the crust on which they lie are both late cretaceous in age this value represents the effective flexural rigidity of very young lithosphere that was frozen in at the time of voicanism the modeling indicates that the general form of a predictive filter will strongly depend on various geologic parameters especially the effective flexural rigidity hence some a priori geologic constraints are necessary to estimate successfully the bathymetry from the altimeter data alternately if high quality bathymetry is available a crude estimate of the age of loading i e voicanism can be made from the altimeter data

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

interpretations of geoid anomalies around the iceland hotspot summary geoid height anomalies around the iceland hotspot in an area extending between 50 w to 14 e and 45 n to 72 n have been investigated to search for evidence of an upper mantle thermal anomaly after removing wavelengths longer than 3800 km the effect of the cooling lithosphere was eliminated by low pass filtering of age data and applying the boundary model the effects of the various platforms and swells in the north atlantic were removed by an airy model since the geoid to topography ratio is lower than 1 5 m km for all areas of elevated topography independent of their origin the residual geoid shows a large positive anomaly with a steep constant slope towards a location north west of iceland beneath greenland the amplitude of the anomaly can be estimated to at least 8m the geoid to topography ratio reaches a value of 7mkm which is in good agreement with a plume related anomaly

geoid heights over the louisville ridge south pacific geoid heights over the louisville ridge seamount chain deduced from the seasat satellite altimeter data have been analyzed and interpreted two uncharted seamounts belonging to the chain have been detected the elastic thickness of the oceanic lithosphere has been determined along the chain this was done rigorously using the three dimensional geoid and bathymetry maps and was also computed approximately using spectral methods on geoid and bathymetry profiles a slightly increasing elastic thickness from the southeast to the northwest in the range 15 5 22 5 km has been estimated

tectonic evolution of the north pacific during the cretaceous quiet period the cretaceous quiet period ended with a significant reorganization of the plate tectonics of the north pacific the farallon plate broke into three primary pieces 1 the kula plate 2 a large fragment transferred to the pacific plate and 3 the continued farallon plate the time of the rearrangement is constrained to just before magnetic anomaly 34 by its presence contrasts in tectonic spreading fabric tsf the emperor trough and the chinook trough our model is based on 1 a complete reevaluation of primary bathymetric and magnetic data 2 the distribution of major troughs emperor chinook hokkaido and krusenstern 3 mapping and analysis of tsf orientation and amplitude on quiet zone crust and 4 available deep sea drilling project drill hole data east west trending magnetic reversal lineations and tsf south of the aleutian trench are bounded on the west by the buldir fracture zone at 175 e the major troughs mark the abandonment and initiation of spreading in the reorganization their absence elsewhere precludes other long distance boundary relocations emperor trough is the abandoned pacific farallon spreading center rift initiation and early slow spreading have created the high amplitude topography of the chinook trough we believe that the hokkaido trough is the initiation site of a failed oceanic rift the incorporation of large amounts of farallon plate into the pacific plate obviates the need for the very rapid pacific farallon spreading rates of previous models and significantly alters the interpretation of mid pacific geology

jasper seamount structure seafloor seismic refraction tomography the velocity structure of jasper seamount was modeled using one and three dimensional inversions of p wave travel times the results represent the first detailed seismic images of a submerged intraplate volcano two seismic refraction experiments were completed on jasper seamount incorporating ocean bottom seismometers and navigated seafloor shots the p wave travel times were first used to compute a one dimensional velocity profile which served as a starting model for a three dimensional tomographic inversion the seamount p velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths this suggests that jasper seamount is constructed predominantly of extrusive lavas with high average porosity the velocity models confirm morphological predictions jasper seamount is a shield volcano with rift zone development high seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks comparisons between p velocity models of jasper seamount and the island of hawaii reveal that these two shield volcanoes are not structurally proportional jasper seamount is far smaller than hawaii yet both volcanoes exhibit an outer extrusive layer of similar thickness this suggests that seamount size influences the intrusive extrusive proportions density equilibrium between melt and country rock may explain this behavior

strontium isotope stratigraphy as a constraint on the age of condensed levels examples from the jurassic of the subbetic zone southern spain condensed levels are often characterized by reworked fossils that may lead to incorrect age assessments strontium isotope stratigraphy is an important chronostratigraphic tool that can be used to verify the biostratigraphic information from condensed beds this paper describes a study of the 87sr 86sr isotope ratios of 56 belemnite samples collected from 28 stratigraphic sections of the boundary between the upper member of the gavila n formation and the zegri formation pliensbachian subbetic zone the petrographic and geochemical data d18o d13c concentrations of fe mn and mg and the sr mn ratio suggest that the belemnites have preserved their original marine geochemical composition after plotting the samples in diagrams of 87sr 86sr values against time according to their biostratigraphic age four different groups a b c and d were obtained with respect to the reference curve in groups a and b the age deduced from the sr isotope ratio is in total or partial agreement respectively with the biostratigraphic age therefore the 87sr 86sr ratio is a good method for the dating correlation and assessment of biostratigraphic results in groups c and d the sis age and the biostratigraphic age do not coincide a graphic procedure is presented as a suitable methodology to constrain the age of the samples showing an sis age that differs from the relative age deduced by biostratigraphy or stratigraphic correlation for the bed they were collected in these situations are interpreted as being the result of reworking of the belemnites group c or ammonites group d that are included in condensed levels these condensed levels formed during the maximum flooding event that led to the drowning of the gavila n carbonate platform the methodology supplied in this paper represents a valuable tool in identifying reworking processes improving correlation and constraining biochronostratigraphic results the values of 87sr 86sr represent a new contribution to the data set of 87sr 86sr ratios for the pliensbachian

the effective elastic lithosphere under the cook austral and society islands abstract we have determined the elastic thicknesste of the oceanic lithosphere along two volcanic chains of the south central pacific cook austral and society islands we used a three dimensional spatial method to model the lithospheric flexure assuming a continuous elastic plate the model was constrained by geoid height data from the seasat satellite along the cook austral chain the elastic thickness increases westward from 2 4 km at mcdonald hot spot to 14 km at rarotonga at mcdonald seamount however the data are better explained by a local compensation model the observed trend shows an increase ofte with age of plate at loading time however the elastic layer under the cook austral appears systematically thinner by several kilometers than expected for normal seafloor suggesting that substantial thermal thinning has taken place in this region considering the apparent thermal age of the plate instead of crustal age improves noticeably the results along the society chainte varies from 20 km under tahiti to 13 km under maupiti which is located 500 km westward when plotting together the society and cook australte results versus age of load we notice that within the first five million years after loading te decreases significantly while tending rapidly to an equilibrium value this may be interpreted as the effect of initial stress relaxation which occurs just after loading inside the lower lithosphere and suggests that the presently measured elastic thickness under the very young tahiti load 0 8 ma is not yet the equilibrium thickness

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

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

estimation of seamount compensation using satellite altimetry the degree of seamount compensation can be estimated from satellite radar altimetry and bathymetry data seamount signatures in the sea surface topography are detected located and extracted from seasat radar altimetry data using a matched filtering algorithm we have developed an efficient method for determining the two dimensional undulation signature of any axi symmetric seamount with an arbitrary degree of compensation given the seamount dimensions as determined from bathymetry we vary its degree of compensation until the model signature matches the signature extracted from seasat data we use this technique to map the relative degrees of local seamount compensation within a region we have applied this technique to 14 of the musicians seamounts two of these seamounts have been analyzed using two crossing tracks each and confirm the repeatability of the technique the results suggest the existence of a chain of low compensation seamounts within the highly compensated musicians independent verification is needed from other geophysical and geochemical analyses to support this hypothesis

early tertiary rupture of the pacific plate 1700 km of dextral offset along the emperor trough line islands lineament abstract between 67 and 40 ma ago a northwest southeast trending fracture system over 8000 km long split the pacific plate and accumulated at least 1700 km of dextral offset between the east and west portions this system here named the emperor fracture zone efz system consisted of several segments one along the present trace of the emperor trough and another along the line islands joined by short spreading ridges the efz terminated at its northern end against the kula pacific ridge and at its southern end in a ridge transform system called the emperor spreading system which extended to the west north of australia the finite angular velocity vector describing the relative motion between the east and west pacific plates is 0 6 ma about a pole at 36 n 70 w this vector added to the known early tertiary motion of the pacific plate with respect to the global hotspot reference frame accounts in large part for the nnw trend of the emperor seamount chain relative to the wnw hawaiian trend without violation of the integrity of the antarctic plate the meiji emperor and emperor hawaiian bends date respectively the initiation 67 ma ago and cessation 40 ma ago of seafloor spreading on the emperor spreading system the postulated early tertiary relative motion along the efz between the east and west pacific plates explains 1 the present misalignment of the two sets of magnetic bights of the pacific 2 the abrupt truncation of eastern pacific bathymetric lineaments against the emperor trough and line islands 3 the contrast in paleolatitude between the eastern and western pacific as indicated by paleomagnetic and sedimentologic studies and 4 the anomalous gravity signature of the central hawaiian ridge that indicates that the ridge loaded thin hot lithosphere

isostatic response of the lithosphere beneath the mozambique ridge sw indian ocean and geodynamic implications summary the sw indian ocean is characterized by the presence of several aseismic features the mozambique ridge an elongated feature lying roughly parallel to the se coast of africa is by far the least known of those structures mainly due to the scarcity of marine data to date the crustal nature and the origin of the ridge are still controversial points since knowledge of the origin of the mozambique ridge is important for a better understanding of the evolution of the sw indian ocean the isostatic response of the lithosphere beneath the ridge is analysed in order to characterize its effective elastic thickness and the emplacement process of the feature two different approaches are applied the direct computation of the geoid anomaly over the ridge by means of a 2 5 dimensional method and the computation of the admittances between the bathymetry and both the geoid and gravity anomalies both approaches point to a local isostatic response of the lithosphere the crustal thickness beneath the mozambique ridge ranges from 17 to 30 km and the average density from 2 78 to 2 7 103 kg m 3 varying with locality in good agreement with the few existing refraction profiles based on our results on the geochemical similarity between the basalts cored at the dsdp site 249 and the mor basalts erlank reid 1974 thompson et al 1982 and on the present knowledge of the sw indian ocean kinematics martin hartnady 1986 we propose an on ridge origin for the mozambique ridge the ridge would have been formed by the anomalous activity of a spreading axis linking the northern mozambique and the transkei basins accretion centres between m10 and m2 times at the m2 epoch a ridge jump event would have caused the spreading to cease the astrid ridge a poorly known aseismic structure lying close to the coast of antarctica near 15 e may be the antarctic counterpart of the mozambique ridge formed by the same accretionary phenomenon the two ridges breaking apart at m2 time due to the ridge jump event however any admittance computation at such high latitudes is impossible at present due to the very poor quality of both bathymetry and altimetry and so this hypothesis cannot be verified

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

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

flexure of the ocean lithosphere from island uplift bathymetry and geoid height observations the society islands summary ln flexure models of the response of the ocean lithosphere to seamount loading three key unknowns must generally be considered the density distribution of the load the degree of fd in of the deflection created moat and the flexural rigidity the models are usually constrained by one of three types of observations gravity or geoicl height anomalies the moat and arch geometry and the amount of uplift of islands on this arch the determination of the model parameters from geoid anomalies is critically dependent on the interpretation of the broad and regional geoid rise that is seen over many of the younger seamount chains and likewise the flexure parameters deduced from the moat and arch geometry are a function of the interpretation of the associated bathymetric swell the uplift data require that the tectonic erosional and sea level history of the region be known although this requirement can be relaxed if rather than the uplift relative to sea level the change in uplift with distance from the load is used as the observed quantity all these types of observations have been used in this paper to constrain the flexure model for the society islands and while any one data type on its own does not lead to reliable flexure parameters a consistent solution is found when the three are taken together the results point to a relatively low load density of 2 5 g m apart from a high density core a moat that is partly fdled in with sediment or volcanic debris and a flexural rigidity of about 3 x dyne cm

guyots of the mid emperor chain mapped with multibeam sonar abstract bathymetry of seven of the middle emperor chain guyots from u s naval oceanographic office multibeam narrow beam swath mapping surveys is presented these data are contoured at a 100 fm interval using different theories on formation juxtaposed with the accurate bathymetry of this chain tectonic possibilities are shown for changes in plate direction and conduit activity fluctuations the range for one complete pulse of this activity is shown to be 280 nm 518 km for the emperor chain