gravity driven thin skinned extension above zechstein group evaporites in the western central north sea an application of computer aided section restoration techniques normal faulting and halokinesis have been important controls on the deposition and subsequent deformation of mesozoic and tertiary strata in the north sea in addition to the previously documented mechanisms of salt withdrawal dissolution and differential sedimentary loading it is recognized that gravity driven thin skinned extension above inclined salt layers has played an important part in north sea basin development commercial section restoration software has been used to facilitate depth conversion restoration and decompaction of seismic sections selected from an interpreted regional database in the western central north sea allowing validation of the interpretations and a graphical and highly quantitative description of salt assisted extension results of this work show that zechstein group evaporites were deposited in shallow sag basins during the permian triassic sedimentary pods were generated by localized deposition in synclinal basins and grabens above the evaporites bedded salt became folded while mobile salt flowed to fill anticline cores since the early jurassic regional tectonic tilting related to post rift subsidence and increasing sedimentary overburden have caused allochthonous mesozoic and tertiary strata to extend by gravity spreading above the mobile salt layer which detaches the allochthon from the underlying autochthonous late palaeozoic rocks concave up listric normal faults sole out in the salt layer propagate into the overlying cover sewuence and have been active at different geological times causing stratal thickening and folding within the allochthon antithetic and synthetic normal faults have developed producing complex upward branching fault systems in map view the listric faults form curvilinear en echelon arrays the faults linked by relay ramps fault blocks are typically 3 7 km wide 2 3 km thick and 7 10 km long salt movement during the jurassic tertiary has been driven by active extension of the cover causing salt to fill potential voids created by fault block rotation thus salt highs occur beneath sites of extension the listric faults generally dip in the same direction as the sub salt surface although there are also some major counter regional faults during extension regional dips have increased up to about 5 which is sufficient for gravity driven extension above a salt layer a total extension of about 6 has occurred the gravity driven thin skinned extension documented in the western central north sea is a phenomenon which can be recognized elsewhere in the north sea basin and can be readily compared with similar phenomena already documented in offshore angola brazil nova scotia and the gulf of mexico

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

the backus gilbert approach to the three dimensional structure in the upper mantle i lateral variation of surface wave phase velocity with its error and resolution summary the backus gilbert method is applied to obtain the phase velocity variations on a sphere from the measured phase velocity narrow peak kernels with radii of about 2000 km are obtained for almost everywhere on the sphere the phase velocity results are thus interpreted as an average within such regions the most trouble comes from the antipodal peak in the resolution kernel this is evaluated as contamination and is incorporated in the error estimation the total error which is a root mean square of contamination from the antipodal peak and statistical error estimated from the data covariance matrix is about 1 per cent of the phase velocity in the average earth model which is the preliminary reference earth model prem however there is about a factor of 2 variation of errors on the sphere maximum variations of phase velocity are about 3 4 per cent of the phase velocity in the average earth model and thus there still remain anomalies which exceed estimated errors the estimated errors correspond to one standard deviation under the assumptions of uncorrelated gaussian distribution for high confidence interval they show that statistically significant anomalies are scarce for the current data set generally love wave phase velocity maps show more resolved features than rayleigh wave maps and we can see in high confidence maps fast velocities in old oceans and old continents and slow velocities in tectonically active regions like the east pacific rise and various back arc regions

thermal and compaction processes in a young rifted basin containing evaporites gulf of lions france the gulf of lions northwestern mediterranean is a young oligocene miocene rifted basin characterized by the presence of a thick evaporitic sequence observed data surface heat flow well temperatures geopressures below the evaporites crustal thinning are used to model 1 the basin forming mechanisms and 2 the intrasedimentary processes that influence the temperature history and thus the maturation of source rocks a two dimensional finite difference basin model temispack simulates the sedimentation compaction and overpressures related to the low permeability of the post rift sequence the heat transfer from the rifted lithosphere into the sediments and the level of maturation by using a kinetic approach the classic passive margin model which relates the history of heat flow and the rate of subsidence to a uniform thinning factor yields subsidence and heat flow predictions that agree with observations the recently accelerated subsidence observed in the deep margin is probably an effect of recent regional compression rather than the consequence of alternative rifting mechanisms the history of maturation of organic matter was influenced by the variation of the regional heat flow due to the rifting the effect of sedimentation or blanketing and the variability of the lithologies and porosities the undercompaction of sediments underlying the evaporites is accounted for by the model which suggests that hydraulic fracturing is taking place at present in the overpressured sediments fluid flow associated with the compaction disequilibrium has negligible thermal effect undercompacted shales below the evaporites have lower thermal diffusivity and thus higher temperatures which has a significant effect on maturation

the tectonic setting of the seychelles mascarene and amirante plateaus in the western equatorial indian ocean abstract a granitic crustal fragment occurs within a system of the marine plateaus in the western equatorial indian ocean the plateaus form an arcuate series of wide shallow banks with small islands in places in spite of the geographical proximity and the morphological similarity the geological structure and origin of the seychelles bank amirante arc and mascarene plateau contrast sharply the seychelles bank is founded on precambrian granite basement the mascarene plateau from saya de malha bank to mauritius island is founded on paleocene basalts and upper cretaceous basalts prevail in the amirante arc geological and geophysical data suggest that the precambrian granite of the seychelles bank is a fragment separated from the indian continental plate during the paleocene this break up was associated with the volcanic activity that built the mascarene plateau the occurrence of middle eocene syenite diorite and microgranite in the western part of the seychelles bank could indicate that crustal accretion across the northwestern indian ocean ridge apparently drove the seychelles block southwestwards causing it to collide with the northeastern edge of the mascarene basin this collision could have led to an early tertiary subduction and subsequently to the development of the amirante trough the absence of post eocene igneous suites in the seychelles could indicate that a change in the spreading pattern of the northwestern indian ocean during the late eocene probably terminated the amirante subduction

correlations between silicic volcanic rocks of the st mary s islands southwestern india and eastern madagascar implications for late cretaceous india madagascar reconstructions abstract the st mary9s islands southwestern india expose silicic volcanic and sub volcanic rocks rhyolites and granophyric dacites emplaced contemporaneously with the cretaceous igneous province of madagascar roughly 88 90 ma ago the st mary9s islands rocks have phenocrysts of plagioclase clinopyroxene orthopyroxene and opaque oxide moderate enrichment in the incompatible elements e g zr 580 720 ppm nb 43 53 ppm la yb n 6 9 7 2 relatively low initial 87 sr 86 sr 0 7052 0 7055 and near chondritic initial 143 nd 144 nd 0 51248 0 51249 they have mineral chemical whole rock chemical and isotopic compositions very close to those of rhyolites exposed between vatomandry ilaka and mananjary in eastern madagascar and are distinctly different from rhyolites from other sectors of the madagascan province we therefore postulate that the st mary9s and the vatomandry ilaka mananjary silicic rock outcrops were adjacent before the late cretaceous rifting that split madagascar from india if so they provide a valuable tool to check and aid traditional cretaceous india madagascar reconstructions based on palaeomagnetism matching precambrian geological features and geometric fitting of continental shelves supplementary material mineral analyses mass balance calculations and locality information are available at http www geolsoc org uk sup18332

quaternary development history of coral reefs from west indian islands a review the western indian ocean results from a complex geological history implying both plate motion since about 180 million years and volcanic hot spot activity since about 68 million years this region exhibits a myriad of islands different in nature and origin including continental remnants madagascar the granitic seychelles high volcanic islands reunion mauritius rodrigues the comoros high and low lying carbonate reef platforms the carbonate seychelles the french scattered island group agalega islands and a few submerged reef banks geyser nazareth saya de malha this region contains all known types of reefs fringing barrier atoll platform and bank the data about reef development history over the quaternary were gained mainly from two distinct sources boreholes and emerged outcrops reef drilling indicates that modern reefs started regionally to grow from about 10 000 9600 years bp rates of vertical reef accretion ranges from 0 2 to about 10 mm year according to exposure and shape of coral builders the sea level curves reconstructed from vertical reef accretion and lagoon deposition curves express a rapid rise in sea level at rates averaging 6 mm year between 10 000 and 7500 7000 years bp with a stabilization at the present position around 2500 3000 years bp there are significant variations in the composition and distribution of dominant reef building coral assemblages and facies framework versus detritus according to exposure to wave energy frameworks composed of robust coral branching assemblages mainly acropora robusta a humilis groups isopora palifera dominate in higher energy settings while those with domal and tabular branching forms mainly porites various faviids acropora hyacinthus group are best represented in lower energy to protected environments detritus reefs are best developed in sheltered sites the recent contribution of some dominant coral species in modern wio reefs seem to have resulted from their delayed arrival in the region as controlled by the regional prevailing surface circulation south equatorial current and agulhas current and or by the competency period of their larvae coral based geochemical records suggest that during the mid holocene the seasonal variability of sea surface temperatures was lower than today the reconstruction of the wio pleistocene reef history is chiefly based on a limited number of exposed reef remains deposited during the last interglacial mis 5 5 stage 129 000 125 000 years bp at present culminating at elevations ranging from 20 to 9 m with respect to present mean sea level such differences in elevation can be explained by a differential tectonic behaviour of reef bearing substrates from stable continental areas to subsiding volcanic islands the existence of older reef generations plio pleistocene was identified from coral rich inclusions trapped into pyroclastites of comoro volcanoes

extension across the indian arabian plate boundary the murray ridge summary seismic reflection profiles from the murray ridge in the gulf of oman northwest indian ocean show a significant component of extension across the predominantly strike slip indian arabian plate boundary the murray ridge lies along the northern section of the plate boundary where its trend becomes more easterly and thus allows a component of extension the dalrymple trough is a 25 km wide steep sided halfgraben bounded by large faults with components of both strike slip and normal motion the throw at the seabed of the main fault on the southeastern side of the halfgraben reaches 1800 m the northwest side of the trough is delineated by a series of smaller antithetic normal faults wide angle seismic gravity and magnetic models show that the murray ridge and dalrymple trough are underlain by a crystalline crust up to 17 km thick which may be continental in origin any crustal thinning due to extension is limited and no new crust has been formed we favour a plate model in which the indian arabian plate boundary was initially located further west than the owen fracture zone possibly along the oman continental margin and suggest that during the oligocene early miocene indian ocean plate reorganization the plate boundary moved to the site of the present owen fracture zone and that motion further west ceased at this time deformation began along the murray ridge with both the uplift of basement highs and subsidence in the troughs tilting the lowest sedimentary unit qalhat seamount was formed at this time subsequent sediments were deposited unconformably on the tilted lower unit and then faulted to produce the present basement topography the normal faulting was accompanied by hanging wall subsidence footwall uplift and erosion flat lying recent sediments show that the major vertical movements have ceased although continuing earthquakes show that some faulting is still active along the plate boundary

geometry of allochthonous chalk group members central trough north sea abstract the temporal and spatial distribution of the hydrocarbon bearing late cretaceous and early tertiary allochthonous chalk is of economic importance examination of cores from the uk sector of the central trough has confirmed the correlation of major allochthonous chalk units to wireline log responses dependent on high porosity allochthonous units were recognized from wireline log coverage of over 100 exploration wells in the central trough area correlation indicated extensive apparent lateral continuity of allochthonous sediment packages enabling the establishment of an event stratigraphy a minimum of nineteen major redeposition events were recognized within the late cretaceous maastrichtian and early palaeocene danian chalks essentially synchronous sourcing and redeposition occurred on both margins of the central trough but with a marked dominance of activity along the eastern margin in terms of incorporated sediment volume the described core material was placed in a regional context by means of the event stratigraphy and confirmed a previously proposed relationship of depositional rock type and transport distance this study recognizes five principal mass flow complexes named after their dominant sediment source areas jaeren north hidra south hidra lindesnes and auk the jaeren north hidra and auk flow complexes are here documented for the first time as are other less extensive deposits

quantitative analysis of north sea subsidence stratigraphic and geophysical maps of the northern and southern north sea basin are used to study mechanisms of intracratonic basin formation these maps include 1 tectonic subsidence for ten stratigraphic units 2 predicted and observed sediment overpressure 3 geographic variation of compaction constants 4 net basement fault extension 5 free air gravity 6 flexural rigidity 7 heat flow and 8 predicted hydrocarbon maturation in northern areas of the north sea observed basement fault extension crustal thinning present day flexural rigidity and tectonic subsidence are predicted reasonably well by a simple stretching model of basin formation with rifting phases during the triassic and mid jurassic early cretaceous in southern areas of the north sea however a similar analysis suggests that other non extensional rift related processes dominated basin evolution the effects of anomalously active aesthenospheric convection coupled with basalt eclogite and deep crustal dewatering reactions are considered as possible additional mechanisms the troughlike form of quaternary tectonic subsidence suggests a new phase of rifting may be starting in the north sea the importance of such thermal rejuvenation is borne out by the association of areas of differential subsidence with high heat flow and low flexural rigidity one component of tectonic reactivation during this period may be due to thermally induced isostatic reequilibration of older tectonic loads

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

seafloor topography and tectonic elements of the western indian ocean the break up of gondwanaland and dispersal of several of its component continental fragments which eventually formed the margins of the indian ocean have produced an ocean basin of enormous variety both in relief and in origin of seafloor features the western half of the indian ocean alone contains every type of tectonic plate boundary both active and fossil and along with some of the deepest fracture zones the most complex mid ocean ridge configurations and some of the thickest sedimentary sequences in the world s ocean basins this ocean is one of the most diverse on the face of the globe we explore the evolution of the morphology of the indian ocean floor and discuss the effect of its variations maxima and minima on the interconnectivity of the ocean s water masses

the tectonic evolution of the indian ocean triple junction anomaly 6 to present the tectonic evolution of the indian ocean ridge ridge ridge triple junction the junction of the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir is inferred by mapping structures along its traces on the three plates with a long range side scan sonar geological long range inclined asdic gloria the sonar images to the west of the triple junction show two different styles of tectonic evolution some sections of the trace on the african plate are marked by a swir facing scarp containing fine lineaments that are probably normal faults these correspond with sections of the trace on the antarctic plate of the same age that have a sharp intersection of swir and seir abyssal hills that is to be expected from a simple ridge ridge ridge triple junction evolution a herringbone pattern of isochrons in contrast there are other sections of the trace on the african plate that have a blocky structure with no definite termination of cir abyssal hills against those of the swir the corresponding regions of the antarctic plate show large 10 to 30 km long overlapping fault scarps tapering to the northeast and ending in asymmetric valleys to the southwest these major normal faults are each offset by 1 6 km forming an en echelon series the two different styles of tectonic structures indicate two different modes of evolution although why the triple junction has evolved with two different modes is unclear periods of asymmetric spreading in the cir magnetic anomaly sequence collected on the african plate may possibly indicate that the cir and seir have been offset by a fracture zone at the triple junction during one of these two modes echo sounder traverses on the antarctic and african plates commonly show a steady shallowing of the cir and seir seafloor toward the traces a steep scarp facing the swir and a broad marginal deep at the base of this scarp this rift flank shape resembles that of the swir valley s rift shoulders close to the present triple junction mitchell 1991a suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes if the shape of the rift shoulder uplift along the trace on the antarctic plate is interpreted with a simple flexural model involving an end loaded elastic plate the inferred rigidity 1019 1021 n m or 2 6 km effective elastic thickness is consistent with the suggestion that the isostatic uplift occurred in young weak lithosphere close to the triple junction interestingly the observed 1 km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load 2 5 1011 n m that is equivalent to half the present cross sectional area of the swir valley 35 km from the triple junction

morphotectonics and mechanics of the central makran accretionary wedge off pakistan abstract swath bathymetry images and 3 5 khz data acquired during rv sonne cruise 123 in autumn 1997 are the base for analysing the morphology and deformation styles of the huge makran accretionary wedge the continental slope off pakistan is characterised by three morphological provinces a the lower slope is built of a sequence of imbricate thrust slices which form long and narrow accretionary ridges with flanks locally as steep as 20 and between 10 and 20 on average while the regional slope is only 1 2 b the nearly flat mid slope terrace is present in the central east part of the margin narrows significantly towards the east and is absent east of 63 45 and c the upper slope up to the shelf has a regionally uniform slope as steep as about 8 and comprises a very rough topography with many gullies and canyons and evidence for mass wasting two meandering canyons crossing the entire margin down to the abyssal plain at about 63 15 63 30 can be correlated with onshore rivers they have been formed by episodic erosion through turbidity flows and are characterised by erosional and depositional portions sinuosity of the canyons is exclusively caused by the accretionary ridges functioning as obstacles for flow to be directly downhill application of coulomb rheology to the frontal part of the makran wedge implies that the mid level decollement is intrinsically extraordinary weak with a great strength contrast to the overlying sediments comparison with nankai cascadia and the western mediterranean ridge reveals new insights in the parameters controlling the shape of an accretionary wedge and the role of a mid level decollement

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

formation of the steer s head geometry of sedimentary basins by differential stretching of the crust and mantle if the lithospheric mantle is stretched over a wider region than the crust postrift stratigraphic onlap will occur at basin margins giving rise to a steer s head geometry space problems are avoided by making the total amount of extension in the lithospheric mantle equal to that in the crust the predicted pattern of onlap agrees well with that observed if the lithospheric mantle is stretched over a fractionally wider region than is the crust thus extensive stratigraphic onlap can be produced easily without recourse to sea level rise or flexural rigidity

physical and biochemical aspects of the flow across the mascarene plateau in the indian ocean this paper presents results from a detailed hydrographic survey of the mascarene plateau and surrounding area undertaken by the rrs charles darwin in june july 2002 we examine how the westward flowing south equatorial current sec crosses the plateau and how the structure of the flow determines the supply of nutrients to the surface waters we find that the flow of the sec across the plateau is highly dependent on the complex structure of the banks which make up the plateau and that a large part of the flow is channelled between the saya de malha and nazareth banks furthermore the sec forms a sharp boundary between subtropical water masses from further south which are low in nutrients and waters from further north which are relatively nutrient rich overall the sec delivers relatively high levels of nutrients to the near surface waters of the central and northern regions of the plateau compared with the southern regions of the plateau this is partly due to uplifting of density surfaces through ekman suction on the northern side of the sec and partly due to the higher levels of nutrients on those density surfaces on the northern side of the sec this may drive increased production of phytoplankton in these areas which would in turn be expected to fuel increased abundances of zooplankton and higher levels of the food chain

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

present day motion along the owen fracture zone and dalrymple trough in the arabian sea all plate motion data available for describing the present day motion between the arabian and indian plates are analyzed to understand the present day tectonic role of the presumed plate boundary the owen fracture zone and dalrymple trough in the northern arabian sea all prior plate motion models are shown to be inconsistent with some of the data along the presumed boundary in particular published global plate motion models predict some convergence across the owen fracture zone and dalrymple trough convergence along the owen fracture zone is contradicted by earthquake focal mechanisms which show right lateral strike slip convergence along the dalrymple trough is contradicted by observed normal faulting in the trough and by normal and strike slip focal mechanisms for earthquakes occurring along or near the trough the sense and rate of motion is further constrained by spreading rates along the sheba ridge northwest of the owen fracture zone and along the carlsberg ridge southeast of the owen fracture zone to estimate the present day motion we determine 43 3 m y average spreading rates from sheba ridge carlsberg ridge and central indian ridge magnetic profiles and have combined them with eight transform azimuths and 15 earthquake slip vectors along these ridges and the arabia india boundary we determine an arabia india euler vector that predicts 2 mm yr of right lateral strike slip motion along the owen fracture zone with a 95 confidence interval of 0 7 mm yr which excludes the faster rates of slips predicted by several prior models the direction of motion is 35 50 counterclockwise from that predicted by published global plate motion models if we omit the data along the arabia india boundary the motion differs insignificantly from zero suggesting that magnetic profiles from the carlsberg and sheba ridges give spreading rates too imprecise to detect the slow arabia india motion if the owen fracture zone and dalrymple trough are regarded as a plate boundary it is the slowest slipping plate boundary known on earth with motion about 100 times slower than the fastest slipping faults along the east pacific rise

a comparison of the moho interpreted from gravity data and from deep seismic reflection data in the northern north sea summary we present the results of gravity modelling of the nsdp 84 deep seismic reflection profiles in the northern north sea the moho depths inferred from forward modelling of marine bouguer gravity anomalies from independent inversion of free air gravity anomalies derived from satellite altimetry and from deep seismic reflection profiling are in good mutual agreement this agreement suggests that the moho discontinuity interpreted from seismic reflection data corresponds to a major jump in density and therefore is likely to represent not only the geophysical but also the petrological boundary between the continental crust and the upper mantle in spite of the significant lateral heterogeneity of the crystalline basement below the northern north sea area as documented by seismic reflection profiling the average density contrast between the basement and the upper mantle is interpreted to show rather little variation this is not true for the deepest part of the viking graben where the nature of the lower crust and upper mantle may have been changed by magmatic processes accompanying the graben formation a relationship between the degree of crustal extension and the volume of magmatic intrusion into the lower crust is tentatively supported by our results

upper mantle p velocity structure of the canadian shield waveform modeling of the phases pnl p and pp is used to determine the p wave upper mantle structure beneath the canadian shield long period recordings at world wide standard seismograph network long range seismic measurement and canadian network stations in north america are used with earthquakes within and just outside the shield as sources the p data set is incomplete due to the scarcity of large events within the shield and the inclusion of pp provides much more data using the wkbj approach to calculate the mantle response the pp waveforms can be modeled almost as well as the p the phase pnl persists to large distances in shield regions and the interference of pnl with triplication range p helps constrain the structure in the upper 400 km important features of our model which is designated s25 are 1 a thick high q high velocity lid to 165 km 2 a deep low velocity zone and 3 discontinuities of 5 at 405 km and 4 at 660 km the lid and low velocity zone create an unusual b branch arrival not seen in other regions which persists well beyond 30 only slight lateral variation is noted within the shield region the major features of the model are similar to those of grand and helmberger s s wave model for the canadian shield