the southern margin of the caribbean plate in venezuela tectono magmatic setting of the ophiolitic units and kinematic evolution abstract the southern caribbean plate margin in venezuela consists of a w e elongated deformed belt composed of several tectonic units dismembered along the northern part of the south america continental plate since the late cretaceous the present review based on petrology and tectono magmatic significance of each unit makes it possible to define the main geotectonic elements and to reconstruct the paleogeographic domains from late jurassic to tertiary a mid ocean ridge basalt morb proto caribbean oceanic basin loma de hierro unit b oceanic plateau dutch and venezuelan islands basement c rifted continental margin cordillera de la costa and caucagua el tinaco units with within plate tholeiitic wpth magmatism d an intra oceanic subduction zone represented by island arc tholeiitic iat magmatism villa de cura and dos hermanas units of early cretaceous age e an early cretaceous ocean continent subduction trench filled by melange franja costera f a new intra oceanic subduction zone represented by the tonalitic arc magmatism of late cretaceous age dutch and venezuelan islands regional tectonic constraints and coherent kinematic reconstruction suggest an original near mid america location of the jurassic cretaceous proto caribbean oceanic realm from early to late cretaceous one sub continental subduction with melanges franja costera unit and two main stages of intra oceanic arc magmatism are recorded in the so called eo caribbean phases the first consists of generally metamorphosed and deformed volcano plutonic sequences with iat affinity villa de cura and dos hermanas units probably in relation to a southeastward dipping subduction the second is mainly represented by generally unmetamorphosed tonalitic intrusives cutting the oceanic plateau in the dutch and venezuelan islands and related to the new intra oceanic subduction with reverse lithospheric sinking the latter probably marked the onset of the aves lesser antilles arc system in the late cretaceous

spatial and temporal variations of subsidence of the east pacific rise 0 23 s abstract the distributions of crustal depths as a function of age have been analysed for the southeast pacific region along the east pacific rise between the equator and the easter microplate 23 s using age data and a new compilation of bathymetric data subsidence rates for both eastern and western flanks asymmetry of subsidence and zero age depths are computed within flow line corridors on the nazca and pacific plates variations of subsidence rates axial depths and subsidence asymmetry are examined both in space within corridors and time within several age intervals the variability in these parameters along the strike of the east pacific rise is systematic and serves to define several orders of ridge segmentation the largest variations of these parameters are correlated with the large scale segmentation of the ridge axis i e transform faults and very large overlapping spreading centres and are interpreted as related to variations in mantle heterogeneities mainly dependent upon temperature smaller variations of subsidence parameters are correlated with second and sometimes third order segmentation of the ridge axis which could be related to variations in axial magmatic supply across strike variations of subsidence suggest the existence of small lateral temperature and density variations in the mantle when analysing the slope of the distribution of depth versus square root of age within corridors we have observed the existence of changes in the slope which occur at specific age limits we have estimated the subsidence over different age ranges in order to determine the temporal evolution of subsidence parameters rates and asymmetry such an analysis may inform on the past axial segmentation and on the persistence of axial discontinuities in time a linear relationship between subsidence rates and axial depths is determined for each age range and suggests that shallower segments subside faster than deeper segments although a similar statistically defined linear relationship exists for any mid ocean spreading ridge both for intermediate or fast ultrafast spreading the resultant slopes of this relationship vary from ocean to ocean and show that this relationship is not universal over all oceans

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

along axis dynamic topography constrained by major element chemistry abstract variations in thickness and density of both the crust and the associated uper mantle have been derived from a compilation of zero age major element composition along the mid atlantic ridge the east pacific rise and the southeast indian ridge assuming isostatic compensation the axial depth computed from major element data correctly agrees with observed axial depth discrepancies are essentially located near hotspots such as iceland and azores the residual topography expressed as the difference between observed and compensated axial depth has a root mean square of 426 m along the three spreading axes which is below the resolution power of the method this insignificant topography which is assumed to contain the dynamic surface topography associated with mantle convection bears an important constraint on the relative variations of the dynamic topography predicted by models of mantle circulation

selected middle ordovician key conodont species from the santa gertrudis formation salta argentina an approach to its biostratigraphical significance

high accuracy high resolution gravity profiles from 2 years of the geosat exact repeat mission satellite altimeter data from the first 44 repeat cycles 2 years of the geosat exact repeat mission geosat erm were averaged to improve accuracy resolution and coverage of the marine gravity field individual 17 day repeat cycles two points per second were first edited and differentiated resulting in alongtrack vertical deflection i e alongtrack gravity disturbance to increase the signal to noise ratio 44 of these cycles were then averaged to form a single highly accurate vertical deflection profile the largest contributions to the vertical deflection error is short wavelength altimeter noise and longer wavelength oceanographic variability the combined noise level is typically 6 rad both types of noise are reduced by averaging many repeat cycles over most ocean areas the uncertainly of the average profile is less than 1 rad 0 206 arcsec which corresponds to 1 mgal of alongtrack gravity disturbance however in areas of seasonal ice coverage its uncertainty can exceed 5 rad to assess the resolution of individual and average geosat gravity profiles the cross spectral analysis technique was applied to repeat profiles individual geosat repeat cycles are coherent 0 5 for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths this emit of resolution is governed by the signal to noise ratio thus when many geosat repeat profiles are averaged together the resolution limit typically improves to about 20 km except in shallow water areas further improvements in resolution will be increasingly difficult to achieve because the short wavelength components are attenuated by upward continuation from the seafloor to the sea surface these results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half wavelength resolution of 12 km by a 4 5 year satellite altimeter mapping mission

gravimetric determination of the continental oceanic boundary of the argentine continental margin from 36 s to 50 s fil arecco maria alejandra universidad de buenos aires facultad de ingenieria argentina

comparison of marine gravity from shipboard and high density satellite altimetry along the mid atlantic ridge 30 5 35 5 s we compare new marine gravity fields derived from satellite altimetry with shipboard measurements over a region of more than 120 000 square kilometers in the central south atlantic newly declassified satellite data were employed to construct free air anomaly maps on 0 05 degree grids sandwell and smith 1992 marks et al 1993 an extensive gravity and bathymetry dataset from four cruises along the mid atlantic ridge from 30 5 35 5os provides a benchmark for testing the two dimensional resolution and accuracy of the satellite measurements where their crosstrack spacing is near their widest the satellite gravity signal is coherent with bathymetry in this region down to wavelengths of 26 km t2 0 5 compared to 12 5 km for shipboard gravity residuals between the shipboard and satellite datasets have a roughly normal distribution the standard deviation of satellite gravity with respect to shipboard measurements is nearly 7 mgal in a region of 140 mgal total variation whereas the internal standard deviation at crossovers for gps navigated shipboard data is 1 8 mgal the differences between shipboard and satellite data are too large to use satellite gravity to determine crustal thickness variations within a typical ridge segment

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

the conodont fauna in the middle ordovician eoplacognathus pseudoplanus zone of baltoscandia conodont faunas from the eoplacognathus pseudoplanus zone in five main areas of middle ordovician rocks in sweden are described and discussed and compared with faunas of comparable age from neighbouring areas the investigation is based on 118 samples which together yielded 175 114 conodont elements the swedish conodont faunas are intermediate between shallower water faunas from the eastern part of baltica and deeper water faunas towards the west correlation with the yangtze platform of south central china is fairly easy and the occurrence of key taxa such as histiodella makes it also possible to correlate some levels with laurentia although faunal lists for the swedish localities are almost identical the conodont taxa have their relative abundance maxima in separate areas species of protopanderodus and periodon are most common in deeper water settings eoplacognathus baltoniodus and microzarkodina characterize intermediate areas and semiacontiodus is typical of shallow subtidal environments less

ordovician conodonts and the tornquist lineament abstract the holy cross mts in southern poland are generally believed to be split by a tectonic dislocation into two separate parts a ne one being a part of the baltic craton and a sw part belonging to the malopolska terrane of a complex geotectonic history connected with the trans european suture zone tornquist lineament unexpectedly conodont assemblages of earliest middle ordovician early darriwilian age from pobroszyn in the northeastern ysogory region and from szumsko in the southwestern kielce region show virtually identical species composition one of the dominant species both in pobroszyn and szumsko trapezognathus pectinatus sp n characterized by denticulated m elements occurs elsewhere only on the northern margin of gondwana separation of the malopolska microcontinent from baltica continued after the disappearance of trapezognathus and an apparently allopatric speciation process was initiated by a population of baltoniodus also in this case denticulation developed in the m elements of the apparatus but the process of speciation of b norrlandicus denticulatus ssp n was truncated by re appearance of the baltic lineage of baltoniodus later conodont faunas from the region are of baltic affinities but remain distinct in showing a relatively high contribution from exotic species of sagittodontina phragmodus and complexodus the periodic north american midcontinent tropical influences observable in the baltic region did not reach the holy cross mts the ordovician conodont faunas from the holy cross mts can be fit into a transect extending from baltica to gondwana with several intermediate terranes this new evidence disproves the long held concept that the ysogory and kielce regions had separate early palaeozoic fates and that the malopolska terrane was unified with baltica already in the earliest ordovician the trans european suture zone truly marks the presence of a wide tornquist sea in the early palaeozoic

fylla bank structure and evolution of a normal to shear rifted margin in the northern labrador sea summary cenozoic seafloor spreading between greenland and north america is generally considered a major right stepping ridge transform ridge system between nw se trending spreading ridge segments in the labrador sea and baffin bay the ridges were linked by n s nne ssw trending transform motions in the davis strait in particular expressed by the 1000 km long ungava fault zone fylla bank part of the southern west greenland continental margin is located in the northernmost labrador sea at the transition between the normal and shear rifting regimes of the labrador sea and davis strait as such the bank may be compared with the demerara plateau part of the french guinea northeast brazil continental margin seismic reflection interpretations presented in this study show that fylla bank is situated above an extensive basin complex herein referred to as fylla structural complex which contains an up to 5 km thick cretaceous cenozoic sedimentary succession above an inferred pre cretaceous basement seismic mapping of basement structures show that the complex is dominated by nnw nw striking rift basins in its southern part and nne striking rift basins in its northern part the rift basins are interpreted to be the result of an initial late early cretaceous rift phase which mainly resulted in the formation of the nnw nw striking structures and a subsequent early campanian rift phase mainly resulting in the formation of large nne striking rotated fault blocks resumed rifting in the early cenozoic deepened the nne striking rift basins the nne oriented structures have previously been interpreted to initiate during the latest cretaceous however this study suggests that they initiated transfer faults already during the late early cretaceous rift phase and possibly correlate with along strike discontinuities in oceanic crust in the labrador sea to define margin segmentation in southern west greenland including the borders of fylla bank a structural kinematic model presented here thus suggests that the cretaceous cenozoic poly phase rifting to some extent was controlled by pre existing crustal fabric combined with an interpreted interplay between normal stresses in the labrador sea and oblique shear stresses in the davis strait this resulted in a very complex structural tectonic evolution and the formation of several distinct structural styles the seismic interpretations are supported by maps of the moho topography and crustal thickness which were compiled from results of pseudo 3 d gravity modelling the maps show minimum crustal thicknesses 11 km and maximum moho uplifts in areas where the nnw nw and nne striking structures interact moreover a strong correlation is found between moho topography crustal attenuation rift enforced thermal uplift and erosion and post rift subsidence in the area this is interpreted to be a result of thermally controlled basin dynamics

altimetry derived gravity predictions of bathymetry by the gravity geologic method the gravity geologic method ggm was implemented for 2 by 2 bathymetric determinations in a 1 6 longitude by 1 0 latitude region centered on the eastern end of the shackleton fracture zone in the drake passage antarctica the ggm used the bouguer slab approximation to process satellite altimetry derived marine free air gravity anomalies and 6 548 local shipborne bathymetric sounding measurements from the korea ocean research and development institute to update the surrounding off track bathymetry the limitations of the bouguer slab for modeling the gravity effects of variable density rugged bathymetric relief at distances up to several kilometers were mitigated by establishing tuning densities that stabilized the ggm predictions tests using two thirds of the shipborne bathymetric measurements to estimate the remaining third indicated that the tuning densities minimized root mean square deviations to about 29 m the optimum ggm bathymetry model honoring all the ship observations correlated very well with widely available bathymetry models despite local differences that ranged up to a few kilometers the great analytical simplicity of ggm facilitates accurately and efficiently updating bathymetry as new gravity and bathymetric sounding data become available furthermore the availability of marine free air gravity anomaly data ensures that the ggm is more effective than simply extrapolating or interpolating ship bathymetry coverage into unmapped regions

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

the early ordovician middle shale member am3 of the amdeh formation and further evidence of conodont faunas from the sultanate of omanap heward et al tremadocian floian shallow marine deposits from oman the middle shale member of the amdeh formation is interpreted to be of early ordovician age based on its trace fossils stratigraphic context and a newly discovered fauna of conodonts the member abruptly overlies the lower quartzite member which may be early cambrian and passes gradationally upward into the upper quartzite member which is probably early middle ordovician the 542 5 m thick middle shale member can be divided into two parts a shaly lower part and a sandy upper part that contains an influx of heavy minerals bioturbation by marine trace fossils is one of the most obvious characteristics of the member the shales and sandstones are interpreted to be of cruziana and skolithos ichnofacies and represent shallow marine shelf shoreface beach and coastal deposits sparse shelly fossils occur in the sandy upper part principally bivalves inarticulate brachiopods ostracods and conodonts the small assemblage of conodonts includes elements interpreted to be tremadocian tetraprioniodus drepanoistodus drepanodus scolopodus tropodus semiacontiodus and teridontus and others which may be floian or ancestral forms of floian taxa balognathidae gen et sp indet a b and aff erraticodon no acritarchs have been recovered probably due to high temperatures experienced during burial to 6 km it is likely that the middle shale member is the seaward equivalent of the mabrouk and barakat formations and an outcrop gamma ray log supports such a correlation the trace fossils sedimentology conodont fauna and the general lack of macrofossils are in keeping with the regional tremadocian floian of the arabian margin of gondwana

satellite altimetry and current meter velocities in the malvinas current at 41 s comparisons and modes of variations three year long current meter arrays were deployed in the malvinas current at 41 s below a satellite altimeter track at about 10 years intervals surface geostrophic velocities sgv derived from satellite altimetric data are compared with the in situ velocities at the upper current meter 300 m multisatellite gridded sgv compare better with in situ observations than along track sgv in spite of the proximity of the moorings to the complex brazil malvinas confluence bmc region satellite sgv are significantly correlated with the 20 day low passed in situ velocities 0 85 for along isobaths velocities 0 8 for cross isobaths velocities the recent in situ measurement period 2014 2015 stands out in the altimetry record with a long lasting 4 months high level of eddy kinetic energy at the mooring site and a southernmost location of the subantarctic front saf the first two modes of variations of sea level anomaly sla over the bmc remarkably match the first two modes of the low passed in situ velocities the first mode is associated with a latitudinal migration of the saf and the second with a longitudinal displacement of the brazil current overshoot the two modes dominate the 24 year long record of sla in the bmc with energy peaks at the annual and semiannual periods for the first mode and at 3 5 months for the second mode the sla over the southwest atlantic was regressed onto the two confluence modes of sla variations and showed remarkable standing wave train like structures in the argentine basin

predicting bathymetry from geosat erm and shipborne profiles in the south atlantic ocean abstract portions of two geosat erm altimeter tracks and corresponding suborbital shipboard gravity and bathymetry profiles in the south atlantic ocean were analyzed one across the walvis ridge about 1100 km long and the other in the brazil basin about 2300 km long together these profiles sample those types of sea floor topography which dominate the gravity signature at wavelengths of 20 to 300 km the walvis ridge is a massive aseismic ridge and the brazil basin profile crosses both an old seamount emplaced at the time the crust was young and a very young mid plate volcano both profiles cross fracture zones after the gravity and bathymetry profiles were split into subprofiles various cross spectral characteristics could be determined by fft techniques analysis showed that observed admittance is not well constrained by either an airy type or flexural compensation models across the walvis ridge but those over the brazil basin can be readily explained by an airy type model with a mean crustal thickness of about 20 km a theoretical filter was then designed based on a priori geological knowledge and used to predict bathymetry from the high passed gravity geoid anomalies not surprisingly the predicted bathymetry shows more detailed and correct short wavelength 20 300 km features than those predicted from the historical data base as represented e g by the dbdb5 gridded bathymetric model for areas where historical shipboard bathymetry measurements are widely spaced longer than about 10 km for single beam data but where some regional geologic information is available such as the relative ages of mid plate volcanoes and crust bathymetry predicted from altimetric data can be used to upgrade regional bathymetrie data bases on which regional geologic geophysical understanding depends

heat flow from the southeast indian ridge flanks between 80 e and 140 e data review and analysis 1 we analyze available heat flow data from the flanks of the southeast indian ridge adjacent to or within the australian antarctic discordance aad an area with patchy sediment cover and highly fractured seafloor as dissected by ridge and fracture parallel faults the data set includes 23 new data points collected along a 14 ma old isochron and 19 existing measurements from the 20 to 24 ma old crust most sites of measurements exhibit low heat flux from 2 to 50 mw m 2 with near linear temperature depth profiles except at a few sites where recent bottom water temperature change may have caused nonlinearity toward the sediment surface because the igneous basement is expected to outcrop a short distance away from any measurement site we hypothesize that horizontally channelized water circulation within the uppermost crust is the primary process for the widespread low heat flow values the process may be further influenced by vertical fluid flow along numerous fault zones that crisscross the aad seafloor systematic measurements along and across the fault zones of interest as well as seismic profiling for sediment distribution are required to confirm this possible suspected effect

antarctic marine gravity field from high density satellite altimetry summary closely spaced satellite altimeter profiles 5 km collected during the geosat geodetic mission geosat gm and those planned for the extended ers 1 mission are easily converted to grids of vertical gravity gradient and gravity anomaly as profile spacing decreases it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross track gradients if one is only interested in the horizontal and vertical derivatives of the gravitational potential however adjustment of the profile is unnecessary the long wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along track derivative of each profile ascending and descending slope profiles are then interpolated onto separate uniform grids these two grids are summed and differenced to form comparable grids of east and north vertical deflection using laplace s equation the vertical gravity gradient is calculated directly from the vertical deflection grids fourier analysis is required to construct gravity anomalies from the two vertical deflection grids these techniques are applied to high density 2 km profile spacing geosat gm profiles in antarctic waters 60 s to 72 s gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean the resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles after adjustment of a dc shift in the shipboard gravity profiles 5 mgal the rms difference between the ship and satellite gravity is 5 5 mgal many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large leaky transform along the pacific antarctic rise

a detailed view of the south pacific geoid from satellite altimetry images of sea surface undulations in the south pacific have been constructed from geos 3 and seasat altimeter data height discrepancies at crossover points associated with long wavelength radial orbit error were suppressed by taking along track derivatives of the ascending and descending profiles these geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical finally the results are displayed by using the hill shading technique where gray tone images represent the innner product of the deflection vector with an assigned sun vector less apparent sea surface undulations can be enhanced by varying the sun s zenith and azimuth shorterwavelength sea surface undulations reflect seafloor topography for instance fracture zones fz s appear as elongated sharp steps in the sea surface while seamounts produce circular bumps since large areas of the south pacific are unsurveyed many previously undetected features appear on the images comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to easter island s expression the dominant features in the images however are the large age offset fz s such as the eltanin and udintsev fz s the images reveal that the eltanin fz is connected to the louisville ridge combined they produce a continuous geoid signature across most of the south pacific this supports the hypothesis of hayes and ewing 1968 that the louisville ridge is the northwest extension of the eltanin fz

location and dynamics of the antarctic polar front from satellite sea surface temperature data the location of the antarctic polar front pf was mapped over a 7 year period 1987 1993 within images of satellite derived sea surface temperature the mean path of the pf is strongly steered by the topographic features of the southern ocean the topography places vorticity constraints on the dynamics of the pf that strongly affect spatial and temporal variability over the deep ocean basins the surface expression of the pf is weakened and the pf meanders over a wide latitudinal range near large topographic features width and temperature change across the front increase and large scale meandering is inhibited elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the pf encounters large topographic features the strong correlations between topography and pf dynamics can be understood in the context of the planetary potential vorticity ppv or f h field mean ppv at the pf varies by more than a factor of 2 along its circumpolar path however at the mesoscale the pf remains within a relatively narrow range of ppv values around the local mean away from large topographic features the pf returns to a preferred ppv value of 25 10 9 m 1 s 1 despite large latitudinal shifts the mean paths of the surface and subsurface expressions of the pf are closely coupled over much of the southern ocean

present and past regional ridge segmentation evidence in geoid data global geoid data from the geosat altimeter satellite and topography data have been analysed in the pacific and atlantic oceans to study present along axis and past off axis regional mid ocean ridge segmentation along the mid atlantic ridge east pacific rise and pacific antarctic ridge axial geoid and depth variations are positively correlated at length scale 800 1000 km respective peak to peak amplitudes are in the range of 0 4 0 8 m and 200 400 m axial variations are correlated with the position of large offset transform faults which coincide with geoid and topography lows triple junctions are also located in geoid and topography lows the observed admittance between geoid and depth is estimated to 2 0 5 m km a value indicative of a dynamic convective origin for the along axis regional variability off axis analysis shows a regional pattern of geoid topography anomalies elongated in the direction of spreading this pattern has also a length scale of 800 1000 km and possibly represents past regional segmentation large offset fracture zones coincide with off axis geoid topography lows

subsurface and deeper ocean remote sensing from satellites an overview and new results satellite remote sensors cannot see far beneath the surface layers of the ocean yet many important ocean processes and features are located well below the surface and at considerable depths examples include mediterranean eddies meddies mixed layer depth internal waves and bottom topography deeper ocean remote sensing is becoming even more important because recent data seem to indicate that the deeper ocean is responding to climate variability and change many of these subsurface phenomena have surface manifestations which can be interpreted with the help of models to derive key parameters of deeper ocean processes the objective of this paper is to provide an overview of satellite remote sensing and modeling techniques which enable scientists to characterize subsurface and deeper ocean processes and features and to present some new results

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

global distribution of seamounts from seasat profiles bathymetrie profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins but only a small fraction of the seafloor has been sampled by ships at the present exploration rate it will take several centuries to map significant portions of the seafloor topography satellite altimetry which maps the topography of the equipotential sea surface is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years using a model of a gaussian shaped seamount loading a thin elastic lithosphere we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile the model predicts that the seamount diameter is equal to the peak to trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed moreover the model suggests that these two measurements are relatively insensitive to the cross track location of the seamount we confirm these model predictions using seasat altimeter profiles crossing 14 well surveyed seamounts in the pacific we then apply the measurement technique to 26 106 million kilometers of seasat profiles resulting in a new global set of seamount locations approximately one quarter of the seamounts identified in seasat profiles were previously uncharted modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid therefore the set of locations is not a straightforward sampling of the total seamount population but is weighted toward seamounts which are poorly compensated a preliminary analysis indicates considerable variations in population density and type across the oceans most notable among them are the absence of seamounts in the atlantic variations in population density across large age offset fracture zones in the pacific the prevalence of small signatures in the indian ocean and the existence of linear trends in the large seamounts of the west pacific

gondwana to asia plate tectonics paleogeography and the biological connectivity of the indian sub continent from the middle jurassic through latest eocene 166 35 ma abstract using the most up to the date information available we present a considerably revised plate tectonic and paleogeographic model for the indian ocean bordering continents from gondwana s middle jurassic break up through to india s collision with asia in the middle cenozoic the landmass framework is then used to explore the sometimes complex and occasionally counter intuitive patterns that have been observed in the fossil and extant biological records of india madagascar africa and eastern eurasia as well those of the more distal continents although the paleogeographic model confirms the traditional view that india became progressively more isolated from the major landmasses during the cretaceous and paleocene it is likely that at various times minor physiographic features principally ocean islands provided causeways and or stepping stone trails along which land animals could have migrated to from the sub continent aside from a likely link albeit broken by several marine gaps to africa for much of this time it is notable that the present day recent biota of madagascar indicates that the ancestors of five land mammal orders plus bats crossed the 400 km wide mozambique channel at different times in the cenozoic it is possible that the kerguelen plateau connected india and australia antarctica in the mid cretaceous approximately 115 90 ma later the seychelles mascarene plateau and nearby elevated sea floor areas could have allowed faunas to pass between southern india and madagascar in the late cretaceous from around 85 65 ma with an early cenozoic extension to this path forming as a result of the reunion hot spot trace islands growing on the ocean floor to the ssw of india the modelling also suggests that india s northward passage towards asia with eventual collision at 35 ma involved the ne corner of the sub continent making a glancing contact with sumatra followed by burma from 57 ma late paleocene onwards a scenario which is compatible with the fossil record indicating that india asia faunal exchanges began occurring at about this time finally we contend that a number of biologically based direct terrestrial migration routes that have been proposed for last 15 m y of the cretaceous asia to india antarctica to madagascar and or india can probably be dismissed because the marine barriers likely varying from 1000 up to 2500 km were simply too wide

thermal evolution of the oceanic lithosphere an alternative view abstract the most common model used for representing the evolution with age of the oceanic lithosphere is the plate model where the temperature is set at a fixed depth called the base of the plate this base of the plate has no physical meaning but this model provides a mathematical substitute for a system where small scale convection occurs through instabilities growing at the base of the cooling lithosphere and becomes effective only below old ocean another possible view is that convection provides heat at the base of the lithosphere whatever the age of the overlying plate this last process can be modeled by a constant heat flow applied on the bottom lithospheric isotherm chablis model a good fit to the observables bathymetry and geoid as function of age and old age heat flow can be obtained both for plate and chablis models in spite of an experimentally determined thermal expansion coefficient much larger than assumed in previous plate models these models have important consequences for several geodynamic processes the plate at an age of 100 ma is only 80 km thick for both models melting above a hot spot can then occur in the garnet spinel transition field without much plate thinning in the plate model the subsidence is stopped at an age of about 80 ma while according to the chablis model several hundred meters of subsidence are expected after 100 ma thus the two models predict quite a different long term pattern of subsidence in the sedimentary basins finally in the chablis model the global cooling of the mantle coming from cold material eroded by secondary convection at the base of the plates is considerably larger than in plate models it amounts to 40 the remaining 60 being due to the subduction process

middle and upper ordovician carbon isotope chemostratigraphy in baltoscandia a correlation standard and clues to environmental history abstract the baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the middle ordovician up to the silurian relatively good preservation of ordovician and silurian carbonate rocks in baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies previous palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the baltoscandian palaeobasin and some of them on different palaeocontinents we propose a composite carbon isotope curve for the baltoscandian region a new subdivision of the middle and upper ordovician succession into seventeen chemostratigraphic zones and an improved nomenclature of isotopic excursions based on analyses of 13 c carb data from 25 drillcore and outcrop sections of estonia latvia and sweden the composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution the katian hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in baltoscandia no abrupt changes occur in the dapingian sandbian part of the baltoscandian isotope curve but the mid darriwilian excursion nearly coincides with the onset of cooler temperatures similar to modern like equatorial sea surface temperatures and related global biodiversification increase suggesting a climatic background for this event

topography of the ocean floor thermal evolution of the lithosphere and interaction of deep mantle heterogeneities with the lithosphere topography associated with large wave length geoid is derived by two methods first the bathymetry corrected for sediment loading is decomposed using a least squares method as a function of age plus terms proportional to the geoid the topography at old ages is flatter predicted by the law at degrees 2 and 3 very little topography is associated with the geoid the transfer function is not even certain in sign but is never smaller than 0 6 more topography seems associated with higher degree harmonics 6 and 7 second the function of age alone best fitting the topography is determined the correlation coefficients of the residual topography with the geoid are then computed a slight correlation appears for degree 6 but not for degrees 2 and 3 these results suggest a mechanical decoupling between the lower mantle mass heterogeneities and the lithosphere

flattening of the oceanic topography and geoid thermal versus dynamic origin summary the origin of the flattening of the seafloor bathymetry and geoid at old ages is still a matter of debate is this departure from the square root of age law induced by dynamic uplift of the lithosphere linked to the flow in the mantle or by cooling of the lithosphere different from that predicted for an infinite half space the amplitude of the dynamic uplift should strongly decrease at short wavelengths and should not be correlated with the seafloor age on a local scale therefore in order to eliminate the dynamic signal the cooling of the lithosphere is studied here by means of depth age and geoid age relationships computed from high pass filtered bathymetric geoid and age fields or from local dtopography d and dgeoid dage slopes the geoid age and depth age relationships obtained here are in good agreement with those previously deduced from global non filtered data at all observation scales a significant departure from the half space solution is found however the data are not sufficiently accurate to discriminate between the plate and chablis constant heat flow at the base of the lithospheric isotherm models our results support the existence of small scale convection supplying heat at the base of the old oceanic lithosphere

an early to middle ordovician succession of conodont faunas at maekalda northern estonia the maekalda section presents a c 12 m thick succession of mainly limestones dolomites and siltstones of late tremadoc to late llanvirn age exposed on the eastern outskirts of tallinn northern estonia the condensed sequence is rich in well preserved conodont elements that provide the basis for detailed biozonation in the baltoscandian zonation beds from the upper varangu paltodus deltifer zone up to the lower aseri eoplacognathus foliaceus subzone have been identified in all the conodont faunas exhibit a close resemblance to swedish faunas many intervals can also be compared with coeval strata in western russia the presence at some levels of representatives of laurentian conodont lineages provides an unusual opportunity for correlating baltoscandian sequences with sequences in north america especially in the lower arenig part of the succession the middle volkhov upper middle arenig paroistodus originalis zone at maekalda is remarkably complete bearing in mind the highly condensed nature of the section as a whole the details of this zone can be correlated with the equivalent intervals in swedish sections in the upper volkhov lower kunda beds there is a considerable stratigraphic gap in the section but the upper kunda has provided an excellent series of successive populations of eoplacognathus indicating that in this part the section is virtually complete

long lived mid ocean ridge segmentation of the pacific antarctic ridge and the southeast indian ridge the pacific antarctic and the southeast indian ridges are examined for evidence of long lived tens of millions of years tectonic segmentation of the mid ocean ridge by examining a variety of geophysical parameters similar to those previously documented in the south atlantic kane and hayes 1992 these parameters include subsidence rate of the oceanic crust individual flanks and asymmetry zero age depth and geoid height decrease with age geoid rate the variability in these parameters along strike of the mid ocean ridge is systematic and serves to define a large scale ridge segmentation of the order of hundreds of kilometers the southeast indian ridge exhibits long lived segmentation in all parameters examined and is subdivided into five flow line corridors with boundaries between corridors occurring at or near major fracture zones while the pacific antarctic ridge exhibits conspicuous asymmetric crustal subsidence evidence of segmentation exists primarily in the along strike geoid rate the observations in these two areas are found to be internally inconsistent with simple thermal conduction models for oceanic crust strongly suggesting that factors other than cooling and isostasy are influencing the creation and subsequent modification of the oceanic crust and lithosphere

modeling hydrography and marine sedimentation in the cariaco basin since the last glacial maximum 1 the cariaco basin has shallow connections with the caribbean sea and these are further reduced at times of lower sea level such as at the last glacial maximum lgm a numerical model was developed to describe the oceanography and biogenic sedimentation in the cariaco basin and nearby caribbean the model is run with different sea levels in order to simulate the changing oceanography and the development of deep water anoxia in the cariaco basin since the lgm in the main sequence of numerical experiments the surface forcing is kept fixed at present day values while the sea level is changed in order to separate the effects of sea level from the effects of climate as the sea level rises the main sedimentation zone moves first to the shallow broad northern sill and ne part of the cariaco basin and then once sea level reaches approximately 60 m below present moves south to the northern coast of mainland venezuela the model shows that there would be an overall increase in sedimentation in the basin as the sea level rises even if there was no change in the surface forcing however the model also shows that sedimentation at particular points in the basin exhibits more complicated behavior which needs to be taken into account when interpreting individual records preliminary numerical experiments examine the effects of changing surface forcing while keeping the sea level at lgm values and the applicability of a mathematical hydraulic control model in this case is also considered

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

bathymetry of the amundsen sea continental shelf implications for geology oceanography and glaciology the amundsen sea continental shelf is one of the most remote areas of coastal antarctica and was relatively unexplored until the late 1980s over the last two decades increased oceanographic and geological interest has led to several cruises that resulted in sufficient bathymetric data to compile a fairly detailed regional map of the amundsen continental shelf we have combined available multibeam and single beam bathymetry data from various sources and created a new regional bathymetry of the amundsen sea continental shelf and margin deep trough systems that dominate the inner shelf are aligned with present glaciers and separated by shallower ridges shaped by paleo ice streams these features merge into a small number of broader troughs on the middle shelf and shoal seaward they now serve as conduits and reservoirs for relatively warm circumpolar deep water this new compilation is a major improvement over previously available regional maps and should aid the numerical modeling of ocean circulation the reconstructions of paleo ice streams and the refinement of ice sheet models

on the interpretation of linear relationships between seafloor subsidence rate and the height of the ridge summary several studies have found that the subsidence rate of the oceanic lithosphere measured along flow lines is a linear function of the height of the ridge this might have very important implications concerning our understanding of the cooling of the lithosphere here we study the relationship between the subsidence rate and the height of the ridge in the south atlantic and east indian oceans from the regression slopes of this relationship it has been inferred that lithosphere created at shallower ridges subsides faster and to deeper levels than that generated at deeper ridges we show however that the topography at young ages for various seafloor segments stays positively correlated with the topography at older ages synthetic data sets are obtained by adding random synthetic signals with the same spectrum as the residual topography to the average depth versus age relationship they are also characterized by the same linear relationship between subsidence rates and the height of the ridge this relationship is therefore not proof that lithosphere formed at shallow ridges cools faster than lithosphere formed at deeper ridges random signals linked with crustal thickness variations hotspot bulges and perhaps large wavelength dynamical topography would also yield such a relationship

arenig middle ordovician ostracods from baltoscandia fauna assemblages and biofacies abstract the arenig ostracod fauna of baltoscandia is the oldest known and amongst the most thoroughly studied ostracod faunas in the world the fauna is dominated by eridostracans and palaeocopes and comprises altogether about fifty species from seven suborders the ten most abundant ostracod species make up 90 of the total arenig fauna overall ostracod diversity estimates in the arenig of the baltoscandian palaeobasin are low but show gradual increase in diversity at younger horizons low diversity may be due to unfavourable climate conditions in the baltoscandian palaeocontinent during the earlier arenig and may also be due to the early stage of evolution of ostracod faunas i e pre their main diversification during the llanvirn thirteen facies related arenig ostracod assemblages are distinguished in the baltoscandian palaeobasin in early and mid volkhov time the assemblages show almost basinwide distribution suggesting many ostracod species were environmental generalists major distinctions between different ostracod biofacies zones can be seen from the late volkhov onwards when the differentiation of ostracod biofacies in the palaeobasin marks the onset of major depth differences ostracod assemblage based reconstruction of sea level changes in the studied area agrees well with the sequence stratigraphic interpretation of the succession and with a sea level curve determined on the basis of sedimentological data

brazil malvinas frontal system as seen from 9 years of advanced very high resolution radiometer data 1 surface thermal fronts in the southwestern atlantic swa ocean are examined using 9 years 1987 1995 of advanced very high resolution radiometer data fronts are detected considering a gradient based edge detector sea surface temperature gradients are calculated from 4 km resolution 5 day composite images covering the western argentine basin south of 30 s variability in the position and intensity of the fronts from seasonal to interannual timescales is characterized in six regions including different parts of the brazil current front bcf and the subantarctic front saf shelf break brazil sb b brazil malvinas collision bm c brazil current overshoot bc o shelf break malvinas sb m malvinas return flow mrf and falkland escarpment fe fronts in the sb b sb m mrf and fe regions are controlled by the bathymetry in the bm c region the bcf and saf appear to merge as a single front this front does not present large seasonal north south excursions as previously described though it pivots seasonally around a fixed point located approximately at 39 5 s 53 5 w changing its orientation from n s in winter to nw se in summer consequently on average the front intersects the 1000 m isobath at 38 30 s in summer and north of 37 s in winter in the bc o region the bcf has a u shape centered at 53 w the intensity of the fronts in each region except in the fe region presents large seasonal variability in the sb b bm c and bc o regions the frontal intensity is highest 0 35 c km during austral winter the annual components explain 83 67 and 71 of the total variance respectively in the sb m and mrf regions the saf is most intense 0 25 c km in summer and fall the annual component of the intensity fluctuations explains 29 and 38 of the total variance respectively in the fe region the annual component of frontal intensity explains only 17 of the variability in the six regions important interannual variability is found the zapiola rise centered at 45 s 43 w appears as a gradient free region closed planetary potential vorticity contours in this area suggest it is dynamically isolated

the cabo de la vela mafic ultramafic complex northeastern colombian caribbean region a record of multistage evolution of a late cretaceous intra oceanic arc ophiolite related rocks accreted to caribbean plate margins provide insights into complex intra oceanic evolution of the plate and its interaction with continental margins of the americas petrologic geochemical and isotope k ar sr and nd data were obtained in poorly known serpentinites gabbros and andesite dykes of the cabo de la vela mafic ultramafic complex from the guajira peninsula north colombia field relations metasomatic alteration patterns and whole rock mineral geochemistry combined with juvenile isotope signatures of the different units suggest that gabbros and serpentinites formed in a slow spreading supra subduction zone that was brought to shallower depths and subsequently evolved to an arc setting where andesitic rocks formed with little sediment input the tectonomagmatic evolution of the cabo de la vela mafic ultramafic complex involved an intra oceanic arc that evolved from pre campanian time to 74 ma relationships with other units from the guajira peninsula show either the existence of a mature arc basement or a series of coalesced allocthonous arcs juxtaposed before accretion onto the passive continental margin of south american in pre eocene times correlations with ophiolite related rocks of the southern caribbean and diachronism in the accretionary process are compatible with a wsw ese advancing caribbean plate front ophiolitic rocks are fundamental to understanding of the complex dynamics and evolution of oceanic crust and its interaction with continental margins to form so called cordilleran type orogens shervais 2001 beccaluva et al 2004 in the caribbean area studies of ophiolitic complexes have identified remnants of rift margins ocean plateaux and arc systems formed in oceanic domains that evolved since the mesozoic following the break up of pangea giunta et al 2002 2006 however different models show lack of concensus testing of regional models through local relationships has often not been successful possibly due to lack of detailed information iturralde vinent lidiak 2006 for discusions a few ophiolite related rocks have been described from the colombian caribbean macdonald 1964 lockwood 1965 alvarez 1967 however their formation setting and their implications for the geotectonic models of the caribbean have not been discussed we present integrated petrological mineral chemistry whole rock geochemistry nd and sr isotopes and k ar geochronology data from the cabo de la vela mafic ultramafic complex of the southwestern caribbean they allow reconstruction of a late cretaceous arc related magmatic system that was accreted to the continental margin of south america in pre eocene times the implication of this and other available regional data is discussed within the framework of the caribbean plate tectonic evolution

detection of seamount signatures in seasat altimeter data using matched filters matched filter techniques are described for the detection of characteristic geoid undulation signatures of seamounts in seasat radar altimeter data the technique requires models of the expected undulation signatures and a statistical description of the background seamount free data examples are given of the detection of 11 known seamounts on four distinct tracks further extensions and applications of the matched filter methods are discussed

high resolution trilobite stratigraphy of the lower middle ordovician oland series of baltoscandia the first ever list of the regional oland series tremadocian to mid darriwilian trilobites and agnostids from the whole of baltoscandia is compiled the study includes revision of systematics as well as vertical and horizontal distribution this is necessary because of the uneven state of knowledge some faunas having being studied recently others not for more than a century three successive faunal types are recognized the olenid fauna surviving from the cambrian in black bituminous shale facies the immigrating ceratopyge fauna and the stabilized asaphid fauna the latter fauna collapsed in the middle of darriwilian and gave way to a set of new post olandian faunas

the open ocean side of the malvinas current in argo floats and 24 years of mercator ocean high resolution 1 12 physical reanalysis downstream of drake passage the northern branches of the antarctic circumpolar current the polar front and the subantarctic front veer northward and the latter forms the malvinas current mc the mc flows along the continental slope up to 38 s where it loops southward as the malvinas return flow using 24 years of mercator ocean physical reanalysis outputs and argo float data we explore the open ocean side of the mc we observe the occurrence of blocking events at 49 s a region where the mc is exposed to the warm and salty anticyclonic anomalies propagating westward along the steep slope of the malvinas escarpment during these events the mc is cut off from its source and the mc transport is considerably reduced at 49 s the open ocean side of the mc is regularly supplied with cold polar waters from the polar front the polar waters accumulate and recirculate between the mc and the malvinas return flow the water characteristics of the recirculation region change over time the recirculation region hosts significantly lighter and fresher waters during the period 1997 2003 compared with prior and later years the 1997 2003 salinity minimum in the recirculation region corresponds to a period with reduced feeding events at 49 s

the foundation seamounts tectonic setting of a newly discovered seamount chain in the south pacific abstract large areas of the south pacific remain unsurveyed because they are so remote a major volcanic chain the foundation seamounts lies between 33 s and 39 s and between 111 w and 125 w poorly defined on conventional bathymetric charts it becomes very apparent when combining an analysis of conventional geophysical data with the satellite altimeter data the length of the seamount chain is 1350 km and the volcanoes are clustered within a 180 km wide band trending approximately n70w which is the direction of absolute motion of the pacific plate over a fixed reference system some of these volcanoes are so shallow that they may have emerged above the sea surface at an earlier time no dates are known for the seamounts surrounding the foundation seamounts chain the tectonic setting is that of the pacific plate at the time when the strike of the spreading ridge changed from a northwesterly direction to the present northeasterly one this episode of spreading reorganization involved the existence of a short lived microplate west of the pacific antarctic ridge a captured segment of nazca plate accounts for the large discrepancies in the magnetic anomalies offsets along the agassiz fracture zone large propagators like the adventure and kurchatov troughs are described the newly discovered western segment of the chile fracture zone is the pacific plate match of the chile fracture zone on the nazca plate

magnetic history of early and middle ordovician sedimentary sequence northern estonia summary alternating field and thermal demagnetization of lime and dolostones from the lower and middle ordovician floian to darriwilian stages subhorizontally bedded sequences in nw and ne estonia reveal two characteristic magnetization components named p and s the intermediate coercivity demagnetized at 30 60 mt up to 300 350 c reversed polarity component p mean of floian stage dref 147 8 10 8 iref 65 8 5 4 combined mean of dapingian and darriwilian stages dref 166 0 8 4 iref 56 1 6 5 is regarded as the primary remanence of early diagenetic chemical origin on the baltica s apparent polar wander path apwp the palaeopoles floian plat 25 0 n plon 50 8 e k 52 7 a95 7 2 dapingian and darriwilian plat 11 4 n plon 39 1 e k 33 8 a95 6 7 are placed on the lower and middle ordovician segment the poles indicate that estonia was located at southerly latitudes decreasing with time floian 48 s dapingian and darriwilian 37 s when the remanence was acquired a high coercivity and high unblocking temperature component s mean of samples dref 33 7 6 3 iref 51 9 5 7 that is regarded as a secondary remanence has both normal and reversed polarities on the european apwp its palaeopole plat 52 5 n plon 157 9 e k 38 9 a95 5 3 gives middle to late permian age according to mineralogical sem and optical microscopy and rock magnetic three component induced remnant magnetization studies component p is carried by magnetite coexisting with glauconite and component s by haematite magnetite is of chemical origin formed in the course of early diagenesis and or dolomitization during the permian continental period haematite the carrier of component s was likely precipitated from oxidizing meteoric fluids in the already existing or simultaneously formed pore space between the dolomite crystals

correlations of mid ocean ridge basalt chemistry with the geoid abstract the major element composition of mid ocean ridge basalts morb shows geochemical variations controlled by the temperature pressure and source composition produced by convection in the mantle beneath the spreading centers the long wavelength signal of the geoid reflects deep mantle processes in this paper we characterize the correlations between the major element chemistry of morb and the geoid anomalies at different wavelengths to determine the extent to which the major element composition of the morb reflects deep mantle circulation the chemical effects of the low pressure fractionation have been corrected using a new method based on the feot mgo ratio the fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the mgo denominator converge to a common point with 8 mgo the separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99 level with the axial depth for both the mid atlantic ridge mar and the east pacific ridge epr and with the geoid only for the mar the latter correlation suggests that the major element composition of the morb is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle the lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the k2o tio2 ratio and the geoid indicates that either source composition does not influence morb chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle

precise location of unsurveyed seamounts in the austral archipelago area using seasat data summary seasat altimetric geoid data are used to detect uncharted seamounts in the austral archipelago area the various physical parameters which affect the geoid signature of a seamount are inspected to analyse their influence on the precision of the location these parameters are the shape of the seamount its density the crustal model the effective elastic thickness of the lithosphere and the distance between the seamount and sub satellite tracks respectively the results of the tests performed on synthetic data as well as on charted structures show that when a seamount is detected on at least two neighbouring tracks it is possible to locate it with good confidence and to give an estimation of its height if the correct elastic thickness is assumed the precision on the location is order 15 km ten previously unsurveyed seamounts have been located in the austral archipelago it appears that they are emplaced along two well defined azimuths n 1 10 e and n 150 e and that two parallel distinct volcanic chains form the austral archipelago

early diversification of ordovician chitinozoans on baltica new data from the jagala waterfall section northern estonia abstract chitinozoans are a group of biostratigraphically valuable microfossils which appeared in the tremadocian and diversified during the rest of the great ordovician biodiversification event to become extinct in the devonian the early diversification of chitinozoans on the baltica palaeocontinent has remained poorly known due to preservation and collecting bias in this study we document the distribution of chitinozoans through the upper tremadocian to lower darriwilian strata in the jagala waterfall section from northern estonia in order to better understand the regional diversity patterns and biogeographic links fifty samples were studied from the 7 5 m succession of sandstones marls and carbonates revealing 47 chitinozoan species and 11 genera the lowermost productive samples attributed to the late tremadocian represent the earliest rich chitinozoan fauna from baltica combined with previous reports this shows a diversity peak in the tremadocian with balanced total diversity reaching 17 possibly followed by a decline in the floian and then a rather gradual increase through the dapingian and early darriwilian the new diversity curve shows more than three times higher values for the tremadocian and dapingian than indicated in previous studies and thus less abrupt radiation in the darriwilian the jagala section provided also new data on the regional biozonal species conochitina cucumis and cyathochitina regnelli which characterize the middle and upper volkhov and lower kunda regional stages respectively several other biostratigraphically significant taxa typical of other regions were recovered from the jagala section including conochitina decipiens and desmochitina bulla a new short ranging species laufeldochitina toilaensis sp nov is introduced having potential to become an index species for middle dapingian strata the early darriwilian assemblage from jagala includes rhabdochitina sp a with up to 2 7 mm long vesicle making it the largest chitinozoan ever reported

mantle dynamics beneath the east pacific rise at 17 s insights from the mantle electromagnetic and tomography melt experiment 1 the electromagnetic data from the mantle electromagnetic and tomography melt experiment are inverted for a two dimensional transversely anisotropic conductivity structure that incorporates a correction for three dimensional topographic effects on the magnetotelluric responses the model space allows for different conductivity values in the along strike cross strike and vertical directions along with imposed constraints of model smoothness and closeness among the three directions anisotropic models provide a slightly better fit to the data for a given level of model smoothness and are more consistent with other geophysical and laboratory data the preferred anisotropic model displays a resistive uppermost 60 km thick mantle independent of plate age except in the vicinity of the ridge crest in most inversions a vertically aligned sheet like conductor at the ridge crest is especially prominent in the vertical conductivity its presence suggests that the melt is more highly concentrated and connected in the vertical direction immediately beneath the rise axis the melt zone is at least 100 km wide and is asymmetric having a greater extent to the west off axis and to the east of the ridge the mantle is more conductive in the direction of plate spreading at depths greater than 60 km the flat resistive conductive boundary at 60 km agrees well with the inferred depth of the dry solidus of peridotite and the deeper conductive region is consistent with the preferred orientation of olivine inferred from seismic observations this suggests that the uppermost 60 km represents the region of mantle that has undergone melting at the ridge and has been depleted of water dissolved hydrogen by contrast the underlying mantle has retained a significant amount of water

analysis of propagators along the pacific antarctic ridge evidence for triggering by kinematic changes abstract we analyze the structure and evolution of two propagators along the pacific antarctic ridge par that we surveyed during the pacantarctic cruise of the n o l atalante a large propagator at 63 30 s 167 w shows a n50 e trending segment of the par propagating southwestward while the adjacent n45 e trending segment retreats the propagating and doomed ridges are offset by about 43 km they both curve into the offset to define an overlap zone about 25 km long the inner pseudofault is juxtaposed to a series of e w trending ridges inferred to represent the failed axes their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure the pseudofaults are 35 5 oblique to the propagating ridge which implies a rate of propagation of 44 8 mm yr using a 62 mm yr full spreading rate comparable to that of other propagators with similar morphology the age of the initiation of the propagation from the heirtzler fracture zone is estimated to be 5 6 ma which coincides with the age of a clockwise change in spreading direction a second smaller southwestward propagator is observed northeast of the major one at 63 15 s 165 10 w with a morphology very similar to that of the larger one it is inferred to have started near 1 ma again at the time of a clockwise change in spreading direction along the par these two propagators are likely to have evolved from extensional relay zones which developed within the heirtzler transform fault tf valley following clockwise changes in spreading direction the present day axial discontinuity is less than 40 km in offset and may not be a tf anymore the development of propagators in this area of the par appears to be triggered by kinematic changes rather than by thermal gradients along the ridge other propagators that have left similar signatures on the flanks of the par appear to have developed at similar spreading rates near 50 60 mm yr full rate as a result of kinematic changes

analysis of observed and predicted tsunami travel times for the pacific and indian oceans i have examined over 1500 historical tsunami travel time records for 127 tsunamigenic earthquakes that occurred in the pacific and indian oceans after subjecting the observations to simple tests to rule out gross errors i compare the remaining reports to simple travel time predictions using huygens method and the long wave approximation thus simulating the calculations that typically take place in a tsunami warning situation in general i find a high correspondence between predicted and reported travel times however significant departures exist some outliers imply significantly slower propagation speeds than predicted many of these are clearly the consequences of observers not being able to detect the possibly weak first arrivals other outliers imply excessively long predicted travel times these outliers reflect peculiar geometric and bathymetric conditions that are poorly represented in global bathymetric grids leading to longer propagation paths and consequently increased travel times analysis of t the difference between observed and predicted travel time yields a mean t of 19 minutes with a standard deviation of 131 minutes robust statistics being less sensitive to outliers yield a median t of just 18 seconds and a median absolute deviation of 33 minutes care is needed to process bathymetry to avoid excessive travel time delays in shallow areas i also show that a 2 2 arc minute grid yields better results that a 5 5 arc minute grid the latter in general yielding slightly slower propagation predictions the largest remaining source of error appears to be the inadequacy of the point source approximation to the finite tsunami generating area

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

geoid height age relation from seasat altimeter profiles across the mendocino fracture zone twenty eight seasat altimeter profiles crossing the mendocino fracture zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age between ages of 15 and 135 m y an unbiased estimate of the overall geoid offset along each profile is determined from a least squares fit of the along track derivative of the geoid to the geoid slope predicted from a simple two layer gravitational edge effect model uncertainties based upon the statistical properties of each profile are also determined a geoid slope age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets the results are in agreement with geoid slope age relations determined from symmetrically spreading ridges sandwell and schubert 1980 however the fracture zone estimates have smaller uncertainties and show less scatter a comparison of these results with the geoid slope age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age 20 40 m y a plate cooling model with a thickness of 125 km is most compatible with the geoid slope age estimates although significant deviations occur these may indicate that the lithospheric thermal structure is not entirely age dependent

comparison of along track resolution of stacked geosat ers 1 and topex satellite altimeters cross spectral analysis of repeat satellite altimeter profiles was performed to compare the along track resolution capabilities of geosat ers 1 and topex data geophysical data records were edited differentiated low pass filtered and resampled at 5 hz all available data were then loaded into three dimensional files where repeat cycles were aligned along track 62 cycles of geosat exact repeat mission 16 cycles of ers 1 35 day orbit 73 cycles of topex the coherence versus wave number between pairs of repeat profiles was used to estimate along track resolution for individual cycles eight cycle average profiles and 31 cycle average profiles geosat and topex only coherence which depends on signal to noise ratio reflects factors such as seafloor gravity amplitude regional seafloor depth instrument noise oceanographic noise and the number of cycles available for stacking averaging detailed resolution analyses are presented for two areas the equatorial atlantic a region with high tectonic signal and low oceanographic noise and the south pacific a region with low tectonic signal and high oceanographic variability for all three altimeters along track resolution is better in the equatorial atlantic than in the south pacific global maps of along track resolution show considerable geographic variation on average globally the along track resolution 0 5 coherence of eight cycle stacks are approximately the same 28 29 and 30 km for topex geosat and ers 1 respectively topex 31 cycle stacks 22 km resolve slightly shorter wavelengths than geosat 31 cycle stacks 24 km the stacked data which are publicly available will be used in future global gravity grids and for detailed studies of mid ocean ridge axes fracture zones sea mounts and seafloor roughness

surveying the flanks of the mid atlantic ridge the atlantis basin north atlantic ocean 36 n new seismic reflection and okean 9 5 khz sidescan sonar data were used to characterise the seismic stratigraphy and structure of the region south of the azores islands north atlantic ocean the new data provide constraints on the tectono sedimentary evolution of deep ocean basins located on the flanks of slow spreading ridges south of the azores thick sedimentary sequences 1 0 s two way travel time correlated with units in the madeira abyssal plain odp sites 950 952 overlay an irregular basement generated by a combination of volcanic and tectonic processes three main cenozoic depositional events are recognised and signed by distinct seismic megasequences the moderate deformation of the sedimentary units and the structural fabric of the underlying oceanic crust shown on seismic and sidescan data suggest the existence of a complex tectonic setting south of the azores illustrating this complex setting is the change from a basement with prominent abyssal hills west of 30 w to a segmented oceanic crust denoting an extensional transtensional tectonic regime east of the latter meridian

comparison of geos 3 and seasat altimeter resolution capabilities a comparison is made between geos 3 and seasat altimeter data with regard to noise power spectral density psd derived geoid psd and short wavelength resolution at wavelengths greater than 450 km the noise psds for geos 3 and seasat are essentially the same and are attributed to mesoscale oceanographic effects at shorter wavelengths the noise spectra diverge because of the higher geos 3 instrument noise the average along track geoid psds are nearly identical between wavelengths of 4500 and 72 km the spectral coherence between repeat tracks shows that the short wavelength resolution of seasat data is 32 km and for geos 3 it is 60 km

origin of the palau and yap trench arc systems summary this paper attempts to account for several unique morphological and geological characteristics of the palau and yap trench arc systems in the northwestern pacific ocean using a new hydrodynamic model postulated herein the model is a simple mathematical treatment of a stationary viscous flow pattern in the asthenosphere that causes an appreciable amount of mechanical resistance against a vertically subducted rigid slab moving horizontally together with the flat plate a couple of thrusting forces comprising uplifting pressure generated in the frontal edge of the overriding lid and a new type of tectonic erosion along the surface of the vertical slab seem to reasonably explain apparently contradictory features of the palau and yap trench arcs long term gps measurements show that the palau and yap trench arcs are moving over the viscous asthenosphere toward the nww with velocities as great as 10 cm yr 1 stresses along the trench arcs predicted by the model would be sufficiently large to maintain their present morphology 1 water depths greater than 8000 m in spite of the extremely small plate convergence rates and relatively young ages of subducted plate 2 v shaped cross section of the axial bottom of these trenches implying little cover of slumped deposits in spite of very steep landward slopes 3 serpentinized peridotites gabbros basalts and limestones are exposed on the landward slope with neither sediment cover nor ferromanganese hydro oxide coatings 4 abnormally close proximity of the palau and yap islands to the trench axis 5 predominant distribution of barrier reefs in their western periphery apparently opposite to the easterly to northeasterly prevailing wind

anatomy of subinertial waves along the patagonian shelf break in a 1 12 global operational model the patagonian slope hosts a variety of waves we used a state of the art ocean reanalysis to examine waves at the shelf break and in the core of the malvinas current mc at periods larger than 10 days statistics over 25 years indicated three types of signals in phase signals at specific locations of the shelf break to the south of 47 s fast propagating signals all along the shelf break phase speed from 140 cm s to 300 cm s at periods between 5 and 110 days and slower signals in the core of the mc phase speeds from 10 cm s to 30 cm s at 20 day 60 day and 100 day periods the large zonal wind stress variations south of 47 s forced in phase along slope velocity variations and triggered fast propagating waves at distinct sites corresponding to abrupt changes in the shelf break orientation the shelf break waves modulated the intensity of the inshore jet which varied from 0 to 30 cm s at 100 m depth and had spatial and temporal structures and scales consistent with those of observed upwelling events slow propagating waves in the core of the mc had along slope wavelengths between 450 and 1200 km and were not forced by the local winds they were tracked back to the drake passage and the malvinas escarpment

observations of the kuroshio s barotropic and baroclinic responses to basin wide wind forcing 1 observations show that the kuroshio in the east china sea ecs kuroshio responds to the large scale wind stress curl field at two time scales it is argued that these two responses are related to barotropic and baroclinic modes that reach the ecs via different waveguides variability in the ecs kuroshio is assessed by comparing satellite altimetry historical hydrography and the pacific decadal oscillation pdo index with the latter used as a proxy for the large scale wind stress curl forcing sea level difference across the ecs kuroshio is positively correlated with pdo at zero lag and negatively correlated at 7 year lag in contrast pycnocline steepness and pdo are uncorrelated at zero lag and negatively correlated at 7 year lag these signals in the ecs kuroshio considered together with wind stress curl anomalies in the open ocean are consistent with a barotropic response to the wind at zero lag the barotropic response is likely forced in the central north pacific by wind stress curl anomalies of opposite sign one of which is centered at ecs latitudes 27 n while the other sits further north this suggests that in general the absolute transport at a given latitude is not simply that predicted by the sverdrup balance along the latitude this is a consequence of waveguides that can steer the barotropic mode across latitude lines in contrast the signals that lag pdo by 7 years are consistent with a baroclinic mode which represents the ocean s time integrated response to the wind stress curl along a single latitude band between 24 n and 27 n

malvinas current volume transport at 41 s a 24 yearlong time series consistent with mooring data from 3 decades and satellite altimetry fil artana camila indira instituto franco argentino sobre estudios del clima y sus impactos argentina centre national de la recherche scientifique francia universite pierre et marie curie francia

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

three dimensional p and s wave velocity structures beneath japan abstract we determined the three dimensional vp and vs structures beneath japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the japan sea and the pacific ocean as well as those at permanent stations on the japan islands as a result we obtained more precise seismic images than previous studies in the crust and the uppermost mantle southwestern honshu exhibited weaker heterogeneity than the other areas in japan corresponding to the distribution of active volcanoes stripe like heterogeneities exist in the subducting pacific slab relatively low velocity zones correspond to low seismicity areas in the pacific slab suggesting that the slab is possibly torn or thin around the areas the fact that nonvolcanic deep tremors associated with the subducting philippine sea slab beneath shikoku kii and tokai do not occur in zones of high vp high vs and low vp vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab prominent and wide low vp and vs zones exist beneath central honshu at the depth range of 30 60 km where the volcanic front related to the subducting pacific plate is located and seismicity around the philippine sea plate is very low this condition may exist because magma genesis processes related to the subducting pacific plate activate the same processes around the philippine sea plate

sensitivity analysis of the gravity geoid estimation a case study on the azores plateau abstract a numerical investigation into the effects of high resolution heterogeneous data on the estimation of the geoid is presented for the area of the azores plateau gravity data were complemented with high resolution digital terrain models mass density models from geophysical information and the most recent geopotential model from the champ mission ggm02 to derive a high precision gravity model gravity data were reduced from the residual terrain model effects and the ggm02c model and a grid with 1 5 spatial resolution was generated by least squares collocation the gravimetric geoid model was computed by the direct evaluation of stokes integral with a spherical cap radius of 0 8 with gravity data reduced by helmert s second condensation method the geoid model was evaluated over sea by comparison with 6 years of topex poseidon altimeter data and an absolute agreement of 14 cm rms was achieved after removing the bias over land the geoid was compared with a total of 223 gps levelling points corresponding to five different vertical datums locally adjusted solutions were determined for each island with a four parameter model and a maximum rms of 12 cm on pico and a minimum of 2 1 cm on faial were determined as a result of persistent volcanic and tectonic activity on most of the azores islands this geoid residual standard deviation must not be viewed as purely geoid error but containing in addition levelling and gps position errors and vertical displacement the effect on geoid accuracy of the digital terrain model resolution of the spatial density of gravity data and of the mass density was evaluated and two important conclusions were derived first the digital terrain model resolution has a clear effect on the geoid accuracy with a maximum rms of 3 4 cm on s miguel second the mass density model does not affect the accuracy of the geoid this second result indicates that the effect of variable density models on geoid determination may apply only to extensive areas of high terrain and not to smaller topographic units such as single volcanoes the high precision gravity model and gravimetric geoid for azores azgeod07 generated in this study will allow the conversion of old orthometric heights to ellipsoidal heights at precisions useful for the monitoring of vertical deformation the geoid will also have application to investigate ocean circulation modulation and dynamic processes of earth s mantle

long lived mid ocean ridge segmentation constraints on models long lived large scale segmentation of the mid ocean ridge as defined by variations in subsidence rate of the ocean crust zero age depth and geoid rate has been documented along three mid ocean ridge systems the south atlantic the pacific antarctic and the southeast indian many of the observations in these three areas are inconsistent with standard thermal models for the generation of oceanic crust and lithosphere three models for subsidence rate and geoid rate and two models for zero age depth are examined sensitivity tests are done to determine how much the thermal and physical constants in these models must vary in order to explain the observations and whether or not such values are plausible only the zero age depth variations can be adequately explained using existing models an internally consistent qualitative model is presented which can explain the data it involves regional and local temperature changes in the mantle both along and across strike of the mid ocean ridge

late cambrian furongian to mid ordovician euconodont events on baltica invasions and immigrations abstract the stratigraphic distributions taxonomy diversity evolutionary lineages and events of late cambrian to middle ordovician euconodonts of baltica are briefly reviewed and re evaluated the euconodonts of the late furongian stage 10 late cambrian to middle ordovician successions of baltica record innovation immigration and two invasions of short duration and one major extinction the innovation event with an origin on baltica is the rise of the proconodontus serratus lineage in the late furongian stage 10 the immigration events comprise the cordylodus immigration to baltoscandia in latest cambrian and this phylogenetic lineage persisted to mid tremadocian early ordovician and vanished at the global mid tremadocian extinction event the late floian basal dapingian transition is a significant event on baltica it is characterized by the immigration of the important genera baltoniodus and microzarkodina and upon their arrival these taxa remained on the baltica palaeocontinent and stayed for a long period forming evolutionary lineages the basal darriwilian immigration event by the genus lenodus and coeval prominent sea level lowstand were caused by a short global icehouse stage related to the expansion of the polar ice sheets the extinction event caused the disappearance of the characteristic cordylodus fauna most events are related to changes in the palaeoclimate resulting from the movement of the baltica palaecontinent and associated eustatic sea level changes however the cause of the global early tremadocian extinction event is not yet completely clear and remains unsolved perhaps abrupt and significant palaeoceanographic changes on a global scale i e plate tectonic movements causing sudden change of ocean currents and climate from warm to cold were the cause of this significant extinction in the early ordovician

autopsy on a dead spreading center the phoenix ridge drake passage antarctica new bathymetric and magnetic anomaly data from the phoenix ridge antarctica show that extinction of all three remaining segments occurred at the time of magnetic chron c2a 3 3 0 2 ma synchronous with a ridge trench collision south of the hero fracture zone this implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision spreading rates slowed abruptly at the time of chron c4 7 8 0 3 ma probably as a result of extinction of the west scotia ridge which would have led to an increase in slip rate and transpressional stress across the shackleton fracture zone spectacular high relief ridges flanking the extinct spreading center mapped for the first time using multibeam swath bathymetry are interpreted as a consequence of a reduction in spreading rate involving a temporary magma oversupply immediately prior to extinction

subsidence of the seafloor in the atlantic and pacific oceans regional and large scale variations we present a deteiled survey of seafloor subsidence over the entire atlantic ocean and in the pacific ocean we examine the evolution of depth as age increases within narrow 200 km wide tectonic corridors elongate in the direction of spreading over the part of the young seafloor where the depth age12 relationship is valid we estimate the subsidence rate by a least squares adjustment in the three oceanic regions west atlantic east atlantic and pacific the subsidence rate shows regional 1000 2000 km and basin scale fluctuations of the order of 100 150 m m y 12 we also note that in some regions subsidence rate and axial depth are correlated with shallow ridges showing fast subsidence and deep ridges slow subsidence on the basis of published models we infer that regional variations in subsidence rate reflect temperature changes in the subaxial mantle of 100 c asymmetrical subsidence is observed in the atlantic especially in the south atlantic the age of departure from the depth age12 linear trend is highly variable with values larger than 100 m y reported in several instances but very low values 20 m y or less characterizing the central pacific flattening predicted by the plate model occurs in some areas in a number of cases however quite distinct behaviour is observed with a sudden depth increase where the depth age12 relationship breaks down and a slow rise of the seafloor at older ages our results suggest that the subsidence behaviour of the seafloor is much more complex than global plate or half space cooling models predict

regional variations in subsidence rate of oceanic plates a global analysis a global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently each plate has been divided into regions bounded by major tectonic features a total of 32 regions have been considered bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region except for three regions western pacific between 10 s and 40 n north american plate between 24 n and 38 n and african plate between 10 n and 24 n where ocean floor older than 80 100 ma flattens with age in other regions depth increases linearly with age1 2 subsidence rate over each region has been computed by linear regression large variations up to a factor 2 are reported from one region to another asymmetrical subsidence is also observed in a number of areas however the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography shallow ridge subsiding quickly and deep ridge subsiding slowly hence seafloor depth subsides at a rate modulated by the value of the initial axial depth this observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid ocean ridges rather than lateral temperature variations inside the asthenosphere involving small scale convection

a rapid method to map the crustal and lithospheric thickness using elevation geoid anomaly and thermal analysis application to the gibraltar arc system atlas mountains and adjacent zones we present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness the main assumptions are local isostasy and a four layered model composed of crust lithospheric mantle sea water and the asthenosphere we consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle we perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of rms error of elevation and geoid anomaly databases the application of this method to the gibraltar arc system atlas mountains and adjacent zones reveals the presence of a lithospheric thinning zone sw ne oriented this zone affects the high and middle atlas and extends from the canary islands to the eastern alboran basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western betics eastern rif rharb basin and gulf of cadiz a number of different even mutually opposite geodynamic models have been proposed to explain the origin and evolution of the study area our results suggest that a plausible slab retreating model should incorporate tear and asymmetric roll back of the subducting slab to fit the present day observed lithosphere geometry in this context the lithospheric thinning would be caused by lateral asthenospheric flow an alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the canary island and extending as far as central europe

eddy variability and energetics from direct current measurements in the antarctic circumpolar current south of australia two year time series measurements of current velocity and temperature in the subantarctic front saf south of australia from 1993 to 1995 provide estimates of eddy fluxes of heat and momentum across the antarctic circumpolar current acc and further insight into the variability of the acc the saf was found to be an energetic meandering jet with vertically coherent fluctuations varying on a timescale of 20 days with typical amplitude 30 cm s21 at 1150 dbar a daily varying coordinate frame that follows the direction of flow allowed mesoscale variability of the saf to be isolated from variability due to meandering of the front and proved very successful for examining eddy fluxes vertically averaged cross stream eddy heat flux was 11 3 kw m 22 poleward and significantly different from zero at 95 for fluctuations in the 2 90 day band zonally integrated this eddy heat flux 50 9 pw is more than large enough to balance heat lost south of the polar front and is as large as cross saf fluxes found in drake passage cross stream eddy momentum fluxes were small not significantly different from zero and of indeterminate sign but tending to decelerate the mean flow a relationship between vertical motion and meander phase identified in the gulf stream was found to hold for the acc eddy kinetic energy levels were similar to those in drake passage and southeast of new zealand eddy potential energy was up to an order of magnitude larger than at other acc sites most likely because meandering of the front is more common here baroclinic conversion was found to be the dominant mechanism by which eddies grow south of australia typical eddy growth time is estimated to be 30 days approximately twice as fast as in drake passage consistent with eddy energy growing rapidly downstream

a theory of the earth hutton and humpty dumpty and holmes paradoxes can be viewed as a bother or as a rich source of information paradoxes fallacies and logical inconsistencies along with the unstated assumptions behind old worldviews often form the foundation for a new worldview this is where science differs from myth and just so stories mantle dynamicists and geochemists have developed a worldview that in many respects is incompatible with modem theories of planetary accretion geodynamics and geophysics and with the geological context of volcanoes a primordial undegassed mantle as both the furnace and the fuel of midplate volcanoes occupies a central role in the geochemical models although modem data alone are sufficient to overturn this view it is interesting to address it in terms of its own assumptions fallacies and inconsistencies and the history of ideas this is a vast field so in honour of james hutton i focus on volcanoes their products their causes and their sources

a global geoid model with imposed plate velocities and partial layering most inversions of the long wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation moreover modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid we have tested whether a good prediction of the geoid can also be achieved by including two constraints a semipermeable behavior of the 660 km discontinuity and surface plate velocities equal to the observed ones the mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km which is proportional to the mass anomaly needed to achieve perfectly layered circulation on the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion the viscosity only varies with depth considering a simple three layer viscosity structure and changing the permeability of the 660 km interface we have obtained a satisfactory variance reduction of the geoid data 75 for degrees 2 12 the best fit to the geoid is obtained if the mass transfer across the 660 km boundary is reduced to one third in comparison with the purely whole mantle model the best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle the amplitudes of dynamic topography predicted by our model are remarkably small 100 m thus fully compatible with the observation

a mean synoptic view of the subantarctic front south of australia abstract a mean synoptic view of the subantarctic front saf is obtained from current meter and hydrographic data by averaging absolute and baroclinic velocity measurements in bins defined by a cross stream coordinate that moves with the current the cross stream coordinate is derived from current meter measurements of vertical shear of the horizontal velocity and is expressed in terms of specific volume anomaly at 780 dbar 780 by averaging absolute and baroclinic velocity measurements in stream coordinates the spatial smoothing that results from eulerian averaging of measurements of a meandering current is avoided the mean saf velocity profile is composed of one central peak and several smaller peaks in the central peak the 2 yr mean absolute velocity from current meters reaches 52 cm s 1 at 420 dbar the mean baroclinic velocity from six ctd sections reaches 34 cm s 1 the saf flow is coherent at all levels reaches the seafloor and is at least 220 km wide the cross stream structure of barocl

conodont biozonation in the upper arenig of sweden conodont faunas from the arenigian lanna volkhov stage in baltoscandia have been the subject of intensive study particularly during the last decade and different biozonations based on them have been proposed the application of some of these zones is discussed here the present investigation is mainly based on 64 samples from ten sections in sweden but conodont collections from bornholm denmark and tamsalu estonia have also been studied lindstrom s 1971 concept of the upper arenigian microzarkodina parva zone is essentially replaced by a revised baltoniodus norrlandicus zone the zone corresponds closely to bagnoli stouge s 1997 recently distinguished b norrlandicus and lenodus antivariabilis zones which can be recognized in sections where a more detailed subdivision and correlation is possible under the names of the t quadrangulum subzone and the l antivariabilis subzone respectively

geography and history of the louisville hotspot chain in the southwest pacific the louisville ridge is a 4300 km long late cretaceous and cenozoic hotspot chain the south pacific equivalent of the hawaiian emperor chain its northwestern end is being consumed by the tonga kermadec subduction zone but seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes of both normal and reversed polarity distributed along a 75 km wide band that can be fitted to small circles about three successive poles of pacific plate hotspot rotation this band obliquely crosses fracture zones of the eltanin system but there is little interaction and no evidence of any genetic connection between the two structures forty of the louisville volcanoes grew above sea level and are preserved as high latitude i e coral free guyots they are spaced less than 100 km apart along most of the chain but there are none within 750 km of the inferred present location of the hotspot beneath a swell at the southeast end of the chain the rate of volcano building by the louisville hotspot declined sharply about 20 m y ago after being remarkably constant at 3 4 103 km3 m y for the previous 50 m y and none of the louisville volcanoes built during the past 10 12 m y the time of most profuse hawaiian volcanism has reached sea level however a seamount from which pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50 5 s 139 2 w guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain and sometimes had an isostatic depression on its loaded crest the depths of guyot shelf breaks increase systematically to the northwest from 530 to 2100 m proportional to the square root of volcano age with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift

magmatic underplating metamorphic evolution and ductile shearing in a mesozoic lower crustal upper mantle unit tinaquillo venezuela of the caribbean belt the tinaquillo orogenic spinel lherzolite is a piece of subcontinental mantle that emplaced in the caribbean belt during late cretaceous it was thrust with an overlying sequence of garnet sillimanite granulites and pyroxene garnet enderbites this crustal formation also contains mafic and ultramafic granulitic rocks similar to those within the peridotite the crust peridotite contact is a several hundred meter thick ductile shear zone parallel to the lithologic units and probably originally flat lying the shearing was coeval with the emplacement of gabbronoritic sills sm nd dated at 90 87 ma macro and microstructures and geothermobarometry show that this tectono magmatic event was preceded by gabbroic intrusions responsible for high temperature metamorphism 800 c and anatexis in the preexisting crustal gneisses u pb zircon dating gives a 125 ma age for the anatexis data suggest that nearly continuous magmatic activity occurred during mid cretaceous times the early gabbroic bodies emplaced at lo

airborne laser altimeter survey of long valley california summary airborne laser altimetry has proved useful in recent years in examining volcanic landforms and glacial ice sheets with the advent of accurate differential gps aircraft tracking we believe that airborne lasers will also prove useful in monitoring timevarying topographic uplift with rates as low as several centimetres of vertical motion per year the long valley california caldera provides an excellent testing ground for this new technology the region has a history of extensive volcanism and its central dome has recently been undergoing resurgent uplift of up to 4 cm per year in september 1993 we conducted three aircraft topographic surveys over the caldera and resurgent dome utilizing a nasa t39 jet aircraft outfitted with a nadir profiling altimetric laser atlas two p code gps receivers a litton ltn92 inertial unit for attitude and aerial cameras in addition we operated two base station gps receivers for post flight differential navigation and conducted a kinematic automobile survey of roads crossing the dome the aircraft flew at a mean altitude of 500m above ground and at speeds of 80 100 m s the laser had a divergence of 1 7 mrad and output 50 pulses per second yielding footprints of 0 9 m diameter separated by about 2 m along track precision flying yielded multiple profiles along nearly identical paths including crossing profiles over the resurgent dome off the dome and along a nearby highway the surveys included daily flights over mono lake for roll and pitch bias calibrations and over the well surveyed lake crowley to provide an independent check of estimated elevations much progress has been made in developing analysis procedures for the laser timing and attitude corrections crossover points from repeated profiles over the dome and the lake indicate that although some profiles contain relative bias errors of up to 10 cm most are substantially more accurate the accuracy of the measurements will increase as the various sources of error are better defined and dealt with even with the present analysis level the distribution of crossovers near the centre of the dome is roughly gaussian with a mean of 2 6 cm and a standard deviation of 11 cm the standard error of the mean is low 1 cm due to the high number of crossover points which holds promise for measuring systematic changes in the dome height from year to year also comparison of laser heights of lake crowley to tidal gauge heights yields only a 1 4 cm difference in absolute height these encouraging results serve to confirm further the concept of using aircraft laser surveys for geodetic tectonic monitoring

the middle lanna volkhov stage middle arenig of sweden and its conodont fauna in the established european conodont zonation the paroistodus originalis zone represents an interval in the upper middle arenig or middle lanna volkhov stage i have so far found it at 14 swedish localities represented in my collections by more than 116 000 conodont elements from 79 limestone samples presence absence data together with major frequency shifts among conodont taxa allow the recognition of five successive phases within the p originalis zone and the corresponding chron phase 1 occurs in the basal part of the zone where an increase in frequency of triangulodus brevibasis and p originalis distinguishes it from immediately subjacent beds phase 2 corresponds to the transgressive lower middle part of the zone with an influx of periodon and drepanoistodus basiovalis replacing d forceps phase 3 is equivalent to the upper middle part of the zone with generally more than 30 of the zonal denominator and with microzarkodina parva replacing m flabellum phase 4 is a short regressive event where the frequency of paroistodus is drastically lowered and phase 5 corresponds to the upper part where paroistodus again increases the zone is particularly well represented at lanna province of narke in south central sweden where it also seems possible to connect the conodont succession with the standard trilobite zonation thus at lanna as well as at finngrundet in the bothnian bay megistaspis m simon first appears in the basal lower middle part of the p originalis zone