three dimensional imaging of the crust using a sparse land acquisition grid the eecors 2 5 d experiment abstract in order to test the feasibility of 3 d imaging of the deep crust at a reasonable cost ecors has designed and conducted an experiment within the western north pyrenean thrust zone using a sparse 3 d land acquisition grid each shot was recorded on 6 parallel geophone lines sampling an 8 12 km2 area and 14 shot columns covered a total surface of 8 27 km2 in a single swath despite the complex structure of the zone standard processing using a bin size of 50 200 m2 provided low fold 3 d stacks on which coherent reflection surfaces could be picked interactively down to 5 s these surfaces which originate from the north pyrenean frontal thrust associated thrust sheets and an uplifted basement wedge have a clearly 3 d geometry the internal structure of the thrust sheets appears to be particularly complex a two pass frequency wavenumber 3 d migration using a dip limitation of 45 and 15 for in and crosslines respectively effectively focused the main reflections and attenuated 2 d migration smiles whereas processing provides a general outline of the complex 3 d structure of the thrust system a detailed interpretation will require an interactive integration of the information present on both original and processed data

a three dimensional study of seismic diffraction patterns from deep basement sources in 1975 the consortium for continental reflection profiling cocorp acquired 37 km of 12 fold common midpoint cmp stacked seismic reflection profiles in hardeman county texas oliver et al 1976 the entire crustal section was the object of study so a record length of 15 sec two way traveltime was recorded three separate profiles were arranged in an h shaped pattern to provide two dimensional 2 d surface coverage in addition to numerous reflections the seismic sections exhibit pronounced apparent diffractions these deep diffractions are more numerous and pronounced here than on any other cocorp survey to date in many cases diffractions on different profiles intersect at the profile junctions if these intersecting diffractions represent the same structural feature observed along different profiles the diffraction time surface provides an estimate of the location of the diffractor and an effective velocity analogous to a stacking velocity to the depth of the diffractor arrival times of these diffraction events were read and a least squares technique was developed to model intersecting diffractions simultaneously and derive location and velocity parameters assuming point or line diffractors nine apparent diffraction events were identified occurring mostly in the 5 9 sec range approximately 15 30 km point diffractor models generally give a closer fit than line models with most of the resulting velocities between 6 0 and 7 5 km sec however the introduction of a radial dimension such as a dome instead of a point significantly lowers these velocities without affecting the quality of fit the locations of the point models when projected to the surface are distributed roughly along a northwest southeast trend the trend is subparallel to the wichita mountains and may indicate deep intrusive and or tectonic structures related to their evolution these results are important evidence that strong heterogeneities on the scale of a few kilometers are present at midcrustal depths

baroclinic transport time series of the antarctic circumpolar current measured in drake passage abstractthe first multiyear continuous time series of antarctic circumpolar current acc baroclinic transport through drake passage measured by moored observations is presented from 2007 to 2011 19 current and pressure recording inverted echo sounders and 3 current meter moorings were deployed in drake passage to monitor the transport during the cdrake experiment full depth acc baroclinic transport relative to the bottom has a mean strength of 127 7 1 0 sverdrups sv 1 sv 106 m3 s 1 with a standard deviation of 8 1 sv mean annual baroclinic transport is remarkably steady about 65 of the baroclinic transport variance is associated with time periods shorter than 60 days with peaks at 20 and 55 days nearly 28 of apparent energy in the spectrum computed from transport subsampled at the 10 day repeat cycle of the jason altimeter results from aliasing of high frequency signals approximately 80 of the total baroclinic transport is carried by the subantarctic front and the polar front partition

circulation in drake passage revisited using new current time series and satellite altimetry 1 the yaghan basin circulation in the yaghan basin

two full depth velocity sections in the drake passage in 2006 transport estimates abstract in january february 2006 a hydrographic section under jason track 104 across drake passage dp was performed twice within 3 weeks on board of the r v polarstern two ladcps mounted on the ctd rosette provided the most comprehensive synoptic top to bottom observations of directly measured velocity available in the drake passage to date with an estimated error of less than 3 cm s 1 geostrophic velocities computed from the hydrographic data were referenced to ladcp velocities the velocity structure of the antarctic circumpolar current acc along the track and its evolution within 3 weeks reflect the frontal branches presented by barre et al 2011 and provost et al 2011 the repeated section offers two independent transport estimates computed from the ladcp velocities 145 8 8 and 137 9 10 5 sv respectively and from the adjusted geostrophic velocities 136 6 7 and 129 7 sv respectively error bars of the transport estimates are small contributions of the fronts and eddies to the total transport are modified within 3 weeks but tend to compensate each other the total transport is reduced by about 10 15 17 sv between the southward and northward journey total transport estimations are compared to previous results in particular the isos international southern ocean study data from which the canonical value of 134 sv has been estimated within the uncertainties due to uneven data distribution no trend is discernible neither in the baroclinic nor total transports from 1975 to 2006

off line acquisition of crustal reflection and refraction data a piggy back seismic reflection survey was carried out in 1984 as part of the dekorp 2 s profile the signal from explosive sources was digitally recorded on 120 channels the objectives were a to record crustal reflections at a wide range of incidence angles 5 85 km offset b to provide 3 d control of deep crustal reflectors seen on the main profile an important consideration throughout the fieldwork was to avoid interference with the main reflection survey this experiment showed that explosive charges as small as 5 90 kg may provide sufficient energy for observations from the steep angle to the wide angle range the off line position of the receivers allowed the reconstruction of steep angle reflection profiles parallel and perpendicular to the main reflection line a strong diffraction cluster on the main line could be correlated laterally and indications about the diffractors shape could be obtained

bottom temperatures in drake passage abstractlong time series of bottom temperatures in the southern ocean are rare the cdrake array with over 40 current and pressure recording inverted echo sounders moored across drake passage to monitor the antarctic circumpolar current acc variability and transport measured temperature at 1 and 50 m above the seafloor at depths 3500 m and at the southern continental margin the 4 yr dataset provided an opportunity to examine the temporal and spatial scales of bottom temperature variability high variability was observed ranges were 0 5 0 9 c in the northern passage and 0 3 0 6 c in the southern passage standard deviations in the two regions were 0 1 0 15 c and 0 05 c respectively meandering of the acc with its deep reaching thermocline accounted for up to 50 of the observed bottom temperature variance northern passage temperatures spaced less than 40 km apart were correlated with each other while those in the southern portion separated by 60 70 km were not a gap in the west scotia

mean antarctic circumpolar current transport measured in drake passage the antarctic circumpolar current is an important component of the global climate system connecting the major ocean basins as it flows eastward around antarctica yet due to the paucity of data it remains unclear how much water is transported by the current between 2007 and 2011 flow through drake passage was continuously monitored with a line of moored instrumentation with unprecedented horizontal and temporal resolution annual mean near bottom currents are remarkably stable from year to year the mean depth independent or barotropic transport determined from the near bottom current meter records was 45 6 sv with an uncertainty of 8 9 sv summing the mean barotropic transport with the mean baroclinic transport relative to zero at the seafloor of 127 7 sv gives a total transport through drake passage of 173 3 sv this new measurement is 30 larger than the canonical value often used as the benchmark for global circulation and climate models

barotropic and baroclinic contributions to along stream and across stream transport in the antarctic circumpolar current the southern ocean s ability to store and transport heat and tracers as well as to dissipate momentum and energy are intimately related to the vertical structure of the antarctic circumpolar current acc here the partition between barotropic and baroclinic flow in the time mean acc is investigated in a southern ocean state estimate the zonal geostrophic transport is predominantly baroclinic with at most 25 of the transport at any longitude carried by the barotropic component following surface streamlines changes in vertical shear and near bottom velocity are large and result in changes in the local partition of barotropic baroclinic vertically integrated transport from 10 90 in the center of the basins to 50 50 near complex topography the velocity at depth is not aligned with the surface velocity this nonequivalent barotropic flow supports significant cross stream transports barotropic and baroclinic mass transport across the acc is on average in opposite directions with the net barotropic cross stream transport being poleward and the net baroclinic equatorward the sum partially cancels out leaving a net geostrophic poleward transport across the different fronts between 5 and 20 sv temperature is also transported across the fronts by the nonequivalent barotropic part of the acc with maximum values across the northern acc fronts equivalent to 0 2 pw the sign and magnitude of these transports are not sensitive to the choice of stream coordinate these cross stream volume and temperature transports are variable in space and dependent on the interactions between deep flow and bathymetry thus difficult to infer from surface and hydrographic observations alone

reconstruction of the central indian ocean abstract sixteen plate tectonic reconstructions are presented illustrating the development of the central indian ocean from the latest cretaceous anomaly 29 to the present the history of seafloor spreading in the indian ocean can be divided into five distinct tectonic epochs 1 1 latest cretaceous early paleocene a29 a27 2 2 middle paleocene early eocene a27 a22 3 3 middle eocene late eocene a22 a18 4 4 late eocene early miocene a18 a8 and 5 5 early miocene recent a8 a5 during each of these tectonic epochs subtle changes took place in the directions and rates of spreading along the central southeast and southwest indian ridges the most dramatic changes in plate motion occurred between a21 and a18 time due to the collision of india and eurasia

upper ocean eddy heat flux across the antarctic circumpolar current in drake passage from observations time mean and seasonal variability abstracteddy heat flux plays a fundamental role in the southern ocean meridional overturning circulation providing the only mechanism for poleward heat transport above the topography and below the

a precambrian microcontinent in the indian ocean ridges of thick raised crust on the indian ocean floor were thought to be mostly volcanic seamounts formed above the reunion mantle plume dating of zircon minerals in mauritian lavas however indicates that fragments of an ancient microcontinent may be preserved beneath the seamounts contributing to the thickened crust

on the variability of antarctic circumpolar current fronts inferred from 1992 2011 altimetry abstractantarctic circumpolar current acc fronts defined as water mass boundaries have been known to respond to large scale atmospheric variabilities especially the southern hemisphere annular mode sam and el nino southern oscillation enso distinct patterns of localized variability in meridional front displacements during 1992 2011 are derived from the analysis of satellite sea surface height data major basin scale differences are found between the southeast pacific 150 90 w and the southeast indian 75 150 e sectors of the acc frontal positions in the southeast pacific show large year to year meridional fluctuations attributed mostly to enso and in part sam and no apparent seasonal cycles or long term trends in contrast summer winter frontal locations in the southeast indian extend farther to the south north of their long term mean distribution a southward drift of acc fronts is indicated over the indian sector during the past two decades this long term shift is not directly re

prevalence of strong bottom currents in the greater agulhas system deep current meter data and output from two high resolution global ocean circulation models are used to determine the prevalence and location of strong bottom currents in the greater agulhas current system the two models and current meter data are remarkably consistent showing that benthic storms with bottom currents greater than 0 2 m s 1 occur throughout the agulhas retroflection region south of africa more than 20 of the time furthermore beneath the mean agulhas current core and the retroflection front bottom currents exceed 0 2 m s 1 more than 50 of the time while away from strong surface currents bottom currents rarely exceed 0 2 m s 1 implications for sediment transport are discussed and the results are compared to atmospheric storms benthic storms of this strength 0 2 m s 1 are comparable to a 9 m s 1 beaufort 5 windstorm but scaling shows that benthic storms may be less effective at lifting and transporting sediment than dust storms

antarctic circumpolar current transport and barotropic transition at macquarie ridge theory and numerical simulations suggest that topographic interactions are central to the dynamics of the antarctic circumpolar current acc but few observations are available to test these ideas we use direct velocity measurements satellite altimetry and an ocean state estimate to investigate the interaction of the acc with the macquarie ridge satellite altimeter data show that the subantarctic front crosses the ridge through a gap immediately north of macquarie island yearlong current meter records reveal strong deep mean flow 0 2 m s 1 at 3000 m and substantial transport 52 8 106 m3 s 1 in the 50 km wide gap the acc becomes much more barotropic at the ridge acceleration of the deep jet is balanced by the ageostrophic along gap pressure gradient convergence of zonal momentum by the mean vertical velocity and dissipation the study helps explain how the acc negotiates large topographic obstacles and highlights the role of local nonlinear processes in the dynamical balance of the acc

the global influence of localized dynamics in the southern ocean the circulation of the southern ocean connects ocean basins links the deep and shallow layers of the ocean and has a strong influence on global ocean circulation climate biogeochemical cycles and the antarctic ice sheet processes that act on local and regional scales which are often mediated by the interaction of the flow with topography are fundamental in shaping the large scale three dimensional circulation of the southern ocean recent advances provide insight into the response of the southern ocean to future change and the implications for climate the carbon cycle and sea level rise

temporal variability of the meridional overturning circulation at 34 5 s results from two pilot boundary arrays in the south atlantic data from two boundary arrays deployed along 34 5 s are combined to produce the first continuous in situ time series observations of the basin wide meridional overturning circulation moc in the south atlantic daily estimates of the moc between march 2009 and december 2010 range between 3 sv and 39 sv 1 sv 106 m3 s 1 after a 10 day low pass filter is applied much of the variability in this 20 month record occurs at periods shorter than 100 days approximately two thirds of the moc variability is due to changes in the geostrophic baroclinic plus barotropic volume transport with the remainder associated with the direct wind forced ekman transport when low pass filtered to match previously published analyses in the north atlantic the observed temporal standard deviation at 34 5 s matches or somewhat exceeds that observed by time series observations at 16 n 26 5 n and 41 n for periods shorter than 20 days the basin wide moc variations are most strongly influenced by ekman flows while at periods between 20 and 90 days the geostrophic flows tend to exert slightly more control over the total transport variability of the moc the geostrophic shear variations are roughly equally controlled by density variations on the western and eastern boundaries at all time scales captured in the record the observed time mean moc vertical structure and temporal variability agree well with the limited independent observations available for confirmation

southern ocean velocity and geostrophic transport fields estimated by combining jason altimetry and argo data zonal geostrophic velocity fields above 1975 dbar have been estimated for the southern ocean from 2004 into 2011 based on sea surface topography observed by jason altimetry and temperature salinity measured by argo autonomous floats the velocity at 1000 dbar estimated with the method has been compared to argo drift trajectory at the same pressure level available from the asia pacific data research center apdrc the inferred velocities agree with those from the argo drift within the estimated sampling error of the latter but have fewer gaps in space and time the velocity has also been integrated from depth to surface to determine the mean and time variable zonal geostrophic transport in the southern ocean between 29 5 s and 58 5 s primarily in the south atlantic and south indian ocean basins due to limitations in coverage of argo analysis shows errors can be reduced by 70 by averaging gridded results over wide areas zonal transport averaged over the entire indian ocean basin shows a significant correlation with the antarctic oscillation aao at low frequencies transport is higher than normal during a positive phase of the aao and lower during the negative phase

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

strong bottom currents and cyclogenesis in drake passage 1 observations from 38 bottom moored current and pressure recording inverted echo sounders cpies deployed in drake passage during the 2007 2008 international polar year provide unprecedented coverage of near bottom currents and pressures spanning the entire antarctic circumpolar current year long mean currents exceed 10 cm s 1 north of the polar front and mean directions are not in general aligned with the surface fronts topographic steering is most evident at the continental margins deep eddy kinetic energy eke is maximum at about 200 cm2 s 2 between the subantarctic and polar fronts coinciding with the location but about one quarter of the value of a maximum in surface eke multiple high speed current events with peak speeds of 60 70 cm s 1 and lasting 30 to 70 days are coherent across sites separated by 45 km the observed spinup of eddies coinciding with meanders in the surface fronts is consistent with deep cyclogenesis

self consistent modeling of crustal thickness at chagos laccadive ridge from bathymetry and gravity data the chagos laccadive ridge clr is an aseismic ridge in the indian ocean that formed by the action of the reunion mantle plume we use modeling of bathymetry and gravity data to constrain the crustal thickness variations at three locations from 0 s to 6 s where the plume was likely interacting directly with the carlsberg spreading center ambiguities in the modeling were reduced by requiring both the density and thickness of the crust to be consistent with a single set of mantle melting conditions in this case using a passive upwelling model in which mantle potential temperature is the controlling factor mean crustal thicknesses beneath the ridge at the three locations predicted from the gravity modeling were 16 6 12 5 and 27 0 km these thicknesses are up to 50 greater than would be obtained had we not required self consistency in the modeling if the thick crust is produced entirely by elevated potential temperatures in the melting region mantle temperatures would have to be 100 250 c higher than normal abrupt decreases in crustal thickness at a large offset fracture zone are consistent with previous models for plume spreading center interactions

circulation in drake passage revisited using new current time series and satellite altimetry 1 the yaghan basin the complex bathymetry of the drake passage and the meridional extent of the shackleton fracture zone in particular force the subantarctic front saf and the polar front pf to veer to the north and the flow of the antarctic circumpolar current concentrates in the yaghan basin we have studied the circulation in the yaghan basin using 3 years of velocity data january 2006 march 2009 at five mooring sites and 18 years of satellite altimetry data mean velocities at our mooring sites show a dominant eastward component which decreases with depth as expected with a notable exception in the center of the yaghan basin where mean velocities reveal a dominant westward component increasing with depth the mooring data suggest the existence of a permanent strong deep cyclonic circulation over the yaghan seafloor depression in the northeastern part of the yaghan basin the in situ data provide the first opportunity to compare altimetry derived velocities with high temporal resolution near surface current meter velocities in a large eddy kinetic energy environment at high latitudes globally altimetry derived velocities compare rather well with the in situ velocities at 500 m depth both in strength and direction correlations are high between the in situ velocities and the surface velocities derived from satellite altimetric data mean sea level estimates lead to reasonable mean surface velocities with however a slight underestimation of the mean velocity at the mean location of the saf on the continental slope and a more important underestimation of the westward current in the center of the yaghan basin a dominant mode of velocity variations 23 of the variance is observed both in the in situ and satellite data corresponding to a strong southward meander of the saf upstream of the mooring line and a northward meander of the pf downstream of the latter the 18 yearlong altimetry time series shows that the mode is robust and has a strong semiannual component

anatomy of the antarctic circumpolar current volume transports through drake passage the 20 year october 1992 to august 2013 observation based volume transport time series of the antarctic circumpolar current acc through drake passage dp across the jason altimeter track 104 is analyzed to better understand the acc transport variability and its potential causes the time series of three transport components total tt barotropic bt and baroclinic bc referenced to 3000 m present energetic intraseasonal fluctuations with a salient spectral peak at 50 and 36 days with the largest least variance being associated with the bt bc component low frequency variations are much less energetic with a significant variance limited to the annual and biannual timescales and show a nonstationary intermittent link with the southern annular mode and the nino 3 4 index for interannual timescales the region around 57 s in the yaghan basin appears to be a strategic point for a practical monitoring of the acc transport as the whole track tt is significantly correlated with the local tt r 5 0 53 and bt r 5 0 69 around 57 s these local bt and tt variations are associated with a well defined tripole pattern in altimetric sea level anomaly sla there is evidence that the tripole pattern associated with bt is locally generated when the bc associated mesoscale slas which have propagated eastward from an upstream area of dp cross the shackleton fracture zone to penetrate into the yaghan basin barotropic basin modes excited within the yaghan basin are discussed as a plausible mechanism for the observed energy containing intra seasonal spectral peaks found in the transport variability

geophysical constraints on understanding the origin of the illinois basin and its underlying crust abstract interpretation of reprocessed seismic reflection profiles reveals three highly coherent layered unconformity bounded sequences that overlie or are incorporated within the proterozoic granite rhyolite province beneath the paleozoic illinois basin and extend down into middle crustal depths the sequences which are situated in east central illinois and west central indiana are bounded by strong laterally continuous reflectors that are mappable over distances in excess of 200 km and are expressed as broad basinal packages that become areally more restricted with depth normal fault reflector offsets progressively disrupt the sequences with depth along their outer margins we interpret these sequences as being remnants of a proterozoic rhyolitic caldera complex and or rift episode related to the original thermal event that produced the granite rhyolite province the overall thickness and distribution of the sequences mimic closely those of the overlying mt simon late cambrian clastic sediments and indicate that an episode of localized subsidence was underway before deposition of the post cambrian illinois basin stratigraphic succession which is centered farther south over the new madrid rift system i e reelfoot rift and rough creek graben the present configuration of the illinois basin was therefore shaped by the cumulative effects of subsidence in two separate regions the proterozoic caldera complex and or rift in east central illinois and west central indiana and the new madrid rift system to the south filtered isostatic gravity and magnetic intensity data preclude a large mafic igneous component to the crust so that any proterozoic volcanic or rift episode must not have tapped deeply or significantly into the lower crust or upper mantle during the heating event responsible for the granite rhyolite

volume transport of the antarctic circumpolar current production and validation of a 20 year long time series obtained from in situ and satellite observations a 20 year long volume transport time series of the antarctic circumpolar current across the drake passage is estimated from the combination of information from in situ current meter data 2006 2009 and satellite altimetry data 1992 2012 a new method for transport estimates had to be designed it accounts for the dependence of the vertical velocity structure on surface velocity and latitude yet unpublished velocity profile time series from acoustic doppler current profilers are used to provide accurate vertical structure estimates in the upper 350 m the mean cross track surface geostrophic velocities are estimated using an iterative error correction scheme to the mean velocities deduced from two recent mean dynamic topographies the internal consistency and the robustness of the method are carefully assessed comparisons with independent data demonstrate the accuracy of the method the full depth volume transport has a mean of 141 sv standard error of the mean 2 7 sv a standard deviation std of 13 sv and a range of 110 sv yearly means vary from 133 6 sv in 2011 to 150 sv in 1993 and standard deviations from 8 8 sv in 2009 to 17 9 sv in 1995 the canonical isos values mean 133 8 sv std 11 2 sv obtained from a year long record in 1979 are very similar to those found here for year 2011 133 6 sv and 12 sv full depth transports and transports over 3000 m barely differ as in that particular region of drake passage the deep recirculations in two semiclosed basins have a close to zero net transport

downstream propagation and remote dissipation of internal waves in the southern ocean recent microstructure observations in the southern ocean report enhanced internal gravity waves and turbulence in the frontal regions of the antarctic circumpolar current extending a kilometer above rough bottom topography idealized numerical simulations and linear theory show that geostrophic flows impinging on rough small scale topography are very effective generators of internal waves and estimate vigorous wave radiation breaking and turbulence within a kilometer above bottom however both idealized simulations and linear theory assume periodic and spatially uniform topography and tend to overestimate the observed levels of turbulent energy dissipation locally at the generation sites in this study we explore the downstream evolution and remote dissipation of internal waves generated by geostrophic flows using a series of numerical realistic topography simulations and parameters typical of drake passage the results show that significant levels of internal wave kinetic energy and energy dissipation are present downstream of the rough topography internal wave generation site about 30 40 of the energy dissipation occurs locally over the rough topography region where internal waves are generated the rest of the energy dissipation takes place remotely and decays downstream of the generation site with an e folding length scale of up to 20 30 km the model we use is two dimensional with enhanced viscosity coefficients and hence it can result in the underestimation of the remote wave dissipation and its decay length scale the implications of our results for turbulent energy dissipation observations and mixing parameterizations are discussed

global rate and spectral characteristics of internal gravity wave generation by geostrophic flow over topography the rate of generation of internal gravity waves in the lee of small length scale topography by geostrophic flow in the world ocean was estimated using linear theory with corrections for finite amplitude topography several global data sets were combined for the calculation including an ocean circulation model for the near bottom geostrophic flow statistics over 500 abyssal current meter records historical climatological data for the buoyancy frequency and two independent estimates of the small scale topographic statistical properties the first topography estimate was based on an empirically derived relationship between paleo spreading rates and abyssal hill roughness with corrections for sedimentation the second estimate was based on small scale 100 km roughness of satellite altimetry derived gravity field using upward continuation relationships to derive estimates of abyssal hill roughness at the seafloor at scales less than approximately 20 km the lee wave generation rate was found to be between 0 34 to 0 49 tw the southern hemisphere produced 92 of the lee wave energy with the southern ocean dominating strength of the bottom flow was the most important factor in producing the global pattern of generation rate except in the indian ocean where extremely rough topography produced strong lee wave generation despite only moderate bottom flows the results imply about one half of the mechanical power input to the ocean general circulation from the extra equatorial wind stress of the world ocean results from abyssal lee wave generation topographic length scales between 176 m and 2 5 km horizontal wavelengths between 1 and 16 km accounted for 90 of the globally integrated generation

the polar front in drake passage a composite mean stream coordinate view the polar front pf is studied using 4 years of data collected by a line of current and pressure recording inverted echo sounders in drake passage complemented with satellite altimetry the location of the pf is bimodal in latitude a northern and southern pf exist at separate times separated geographically by a seafloor ridge the shackleton fracture zone and hydrographically by 17 cm of geopotential height expressed in stream coordinates vertical structures of buoyancy are determined with a gravest empirical mode analysis baroclinic velocity referenced to zero at 3500 dbar width and full transport about 70 sv of the jets are statistically indistinguishable the two jets alternate carrying the baroclinic transport rather than coexisting influences of local bathymetry and deep cyclogenesis manifest as differences in deep reference velocity structures downstream reference velocities of the pf n and pf s reach maximum speeds of 0 09 and 0 06 m s 1 respectively buoyancy fields are indicative of upwelling and poleward residual circulation at the pf based on potential vorticity and mixing lengths the northern and southern pf both act as a barrier to cross frontal exchange while remaining susceptible to baroclinic instability

jet topography interactions affect energy pathways to the deep southern ocean abstractin the southern ocean strong eastward ocean jets interact with large topographic features generating eddies that feed back onto the mean flow deep reaching eddies interact with topography where turbulent dissipation and generation of internal lee waves play an important role in the ocean s energy budget however eddy effects in the deep ocean are difficult to observe and poorly characterized this study investigates the energy contained in eddies at depth when an ocean jet encounters topography this study uses a two layer ocean model in which an imposed unstable jet encounters a topographic obstacle either a seamount or a meridional ridge in a configuration relevant to an antarctic circumpolar current frontal jet the authors find that the presence of topography increases the eddy kinetic energy eke at depth but that the dominant processes generating this deep eke depend on the shape and height of the obstacle as well as on the baroclinicity of the jet before it encounters topography i

reconciling plate kinematic and seismic estimates of lithospheric convergence in the central indian ocean the far field signature of the india asia collision and history of uplift in tibet are recorded by sediment input into the indian ocean and the strain accumulation history across the diffuse plate boundary between the indian and capricorn plates we describe the history of india capricorn convergence from updated estimates of india somalia capricorn plate rotations and observations derived from seismic reflection data new india capricorn plate rotations for the past 20 m y are consistent with slow north south convergence from 18 ma about a stationary or nearly stationary pole near the eastern edge of the chagos laccadive ridge simpler than predicted by previous models based on many fewer data the new rotations suggest that convergence began between 18 and 14 ma consistent with marine seismic evidence for an onset of deformation at 15 4 13 9 ma they further show that convergence rates doubled at 8 ma in agreement with a sharp increase in fault activity at 8 7 5 ma seen on seismic reflection profiles a discrepancy between the total strain estimated from kinematic and seismic reflection data can be reconciled if pervasive reverse faulting within the diffuse plate boundary is accompanied by block rotations of 1 3

mean dynamic topography in the southern ocean evaluating antarctic circumpolar current transport 1 mean dynamic ocean topography mdt is the difference between the time averaged sea surface height and the geoid combining sea level and geoid measurements which are both attained primarily by satellite is complicated by ocean variability and differences in resolved spatial scales accurate knowledge of the mdt is particularly difficult in the southern ocean as this region is characterized by high temporal variability relatively short spatial scales and a lack of in situ gravity observations in this study four recent southern ocean mdt products are evaluated along with an mdt diagnosed from a southern ocean state estimate mdt products differ in some locations by more than the nominal error bars attempts to decrease this discrepancy by accounting for temporal differences in the time period each product represents were unsuccessful likely due to issues regarding resolved spatial scales the mean mass transport of the antarctic circumpolar current acc system can be determined by combining the mdt products with climatological ocean density fields on average mdt products predict higher acc transports than inferred from observations more importantly the mdt products imply an unrealistic lack of mass conservation that cannot be explained by the a priori uncertainties mdt estimates can possibly be improved by accounting for an ocean mass balance constraint

absolute geostrophic velocity within the subantarctic front in the pacific ocean velocity measurements from a shipboard acoustic doppler current profiler adcp are used as a reference for geostrophic current calculations on six sections across the subantarctic front saf in the pacific ocean the resulting cross track velocity estimates near the bottom range from 4 to 10 cm s 1 to the east in the eastward jet at the saf in adjacent regions of westward surface flow the near bottom velocity is usually to the west on one section where simultaneous lowered adcp velocity profiles are available they confirm the results from the shipboard adcp annual mean velocity sections from the parallel ocean program numerical model also show near bottom velocities exceeding 5 cm s 1 with the same tendency for the zonal velocity component near the bottom to match the direction of the surface jets transport across the entire antarctic circumpolar current acc cannot be estimated accurately from adcp referenced geostrophic sections because even a very small cross track bias integrates to a large error a preliminary look at the 1992 model transport stream function shows that the effect of bottom referencing varies from section to section it can cause 40 sv recirculations to be missed and can cause net transport to be underestimated or overestimated by o 30 sv

3d modelling of diffractions observed on deep reflection line dekorp 2 s1 dekorp 2 s is the first profile carried out in the german continental reflection seismic programme besides numerous reflections in the lower crust the seismic section is characterized by strongly curved events that are interpreted as diffractions these diffractions occur as clusters mainly in two areas of the profile the dinkelsbuhl and the spessart area this paper deals with the dinkelsbuhl diffractions where three dimensional control is available the control is provided by two additional profiles p 1 and q 40 which run parallel and perpendicular to the main line dekorp 2 s the type and the location of the diffractors are determined by traveltime modelling using crustal velocity functions derived from in line wide angle observations a model with inclined line diffractors provides the best fit to the data for all three profiles projections of these line diffractors to the surface show that they are aligned parallel to the strike direction of the variscides this suggests that the diffractions are associated with the suture zone between the saxothuringian and moldanubian geological provinces

application of revised ray tracing migration to imagine lateral variations of seismic fabric corresponding to different tectonic styles in the northern apennines abstract migration of zero offset seismic sections of deep crust can be done with methods based on ray tracing we modify the classical ray tracing migration method rtm introducing a consistency check to control whether back propagated rays satisfy the condition of strict normal incidence at the migrated reflector a synthetic test shows the effectiveness of the method in particular the control of normal incidence allows elimination of physically inconsistent reflectors from the migrated section then rtm is applied to a crustal seismic profile acquired in central italy using a velocity model obtained from wide angle data that reproduces the gross structures of the apenninic crust the lateral variation of the seismic fabric shown from the migrated section reveals the presence of coexisting extensional and compressional tectonic regimes kinematic diffraction modelling gives additional information about both the distribution of seismic velocities and major active geodynamic processes in the upper lithosphere the migrated section supports the subdivision of northern apennines in two tectonic regions a stretched upper plate tuscany and northern thyrrenian supported by a rise of the asthenosphere and a downwarped lower plate adria subducted below the mountain belt

crustal shortening and duplication of the moho in the northern apennines a view from seismic refraction data abstract a reappraisal of the dss refraction seismic campaigns of 1978 and 1974 in the northern apennines and the northern tyrrhenian sea after digitization of the original analog data and a new interpretation of crustal structures are presented in this paper the layering of the adria crust consists of a low velocity lower crust topped by a 6 7 km s horizon and an upper crust again formed by a low velocity layer capped by a faster one in the tuscany sector the crust mantle boundary and lower crust show attenuated velocities and a reduced thickness with respect to the adriatic counterpart these differences are due to the extensional tectonics restricted to tuscany and occurred in a high heat flow regime related to an uplifting astenosphere it has been confirmed that a deep thrust is responsible for moho doubling in correspondence with the zero bouguer anomaly line the tuscan moho overlaps the umbro marchean one for a length of about 30 km it is hypothesized that another shallower thrust involves upper crust in the mid tuscany range mt cetona and perugia massifs had their roots in the mantle and that subsequent extensional tectonics obliterated every trace of thrusting in lower crust and mantle rocks no conclusive proof of the existence of a european moho below a tuscan one in the area underneath elba has yet been found revision of dss data excludes the presence of a refractor reflector 60 km deep the same data however are compatible with the existence of a crust mantle boundary at 30 35 km depth

moho depth variations over the maldive ridge and adjoining arabian and central indian basins western indian ocean from three dimensional inversion of gravity anomalies abstract analysis of high resolution satellite derived free air gravity data has been undertaken in the greater maldive ridge gmr maldive ridge deep sea channel northern limit of chagos bank segment of the chagos laccadive ridge and the adjoining arabian and central indian basins a complete bouguer anomaly cba map was generated from the indian ocean geoidal low removed free air gravity hereinafter referred to as fag iogl data by incorporating bullard a b and c corrections using the parker method moho topography was initially computed by inverting the cba data from the cba the mantle residual gravity anomalies mrga were computed by incorporating gravity effects of sediments and lithospheric temperature and pressure induced anomalies further the mrga was inverted to get moho undulations from which the crustal thickness was also estimated it was found that incorporating the lithospheric thermal and pressure anomaly correction has provided substantial improvement in the computed moho depths especially in the oceanic areas but along the gmr there was not much variation in the moho thickness computed with and without the thermal and pressure gravity correction implying that the crustal thickness of the ridge does not depend on the oceanic isochrones used for the thermal corrections the estimated moho depths in the study area ranges from 7 km to 28 km and the crustal thickness from 2 km to 27 km the moho depths are shallower in regions closer to central indian ridge in the arabian basin i e the region to the west of the gmr is thinner compared to the region in the east central indian basin the thickest crust and the deepest moho are found below the n s trending gmr segment of the chagos laccadive ridge along the gmr the crustal thickness decreases from north to south with thickness of 27 km below the maldives ridge reducing to 9 km at 3 s and further increasing towards chagos bank even though there are similarities in crustal thickness between maldive ridge and other regions like mascarene plateau which was recently interpreted as underlain by continental crust much more geoscientific work including drilling has to be undertaken to finally confirm the exact nature of the ridge

diffraction imaging of sub vertical fractures and karst with full resolution 3d ground penetrating radar abstract vertical fractures with openings of less than one centimetre and irregular karst cause abundant diffractions in ground penetrating radar gpr records gpr data acquired with half wavelength trace spacing are uninterpretable as they are dominated by spatially undersampled scattered energy to evaluate the potential of high density 3d gpr diffraction imaging a 200 mhz survey with less than a quarter wavelength grid spacing 0 05 m 0 1 m was acquired at a fractured and karstified limestone quarry near the village of cassis in southern france after 3d migration processing diffraction apices line up in sub vertical fracture planes and cluster in locations of karstic dissolution features the majority of karst is developed at intersections of two or more fractures and is limited in depth by a stratigraphic boundary such high resolution 3d gpr imaging offers an unprecedented internal view of a complex fractured carbonate reservoir model analogue as seismic and gpr wave kinematics are similar improvements in the imaging of steep fractures and irregular voids at the resolution limit can also be expected from high density seismic diffraction imaging

vertical structure and transport of the antarctic circumpolar current in drake passage from direct velocity observations 1 the structure of the antarctic circumpolar current acc in drake passage is examined using 4 5 years of shipboard acoustic doppler current profiler adcp velocity data the extended 1000 m depth range available from the 38 khz adcp allows us to investigate the vertical structure of the current the mean observed current varies slowly with depth while eddy kinetic energy and shear variance exhibit strong depth dependence objectively mapped streamlines are self similar with depth consistent with an equivalent barotropic structure vertical wavenumber spectra of observed currents and current shear reveal intermediate wavenumber anisotropy and rotation indicative of downward energy propagation above 500 m and upward propagation below 500 m the mean observed transport of the acc in the upper 1000 m is estimated at 95 2 sv or 71 of the canonical total transport of 134 sv mean current speeds in the acc jets remain quite strong at 1000 m 10 20 cm s 1 vertical structure functions to describe the current and extrapolate below 1000 m are explored with the aid of full depth profiles from lowered adcp and a 3 year mean from the southern ocean state estimate sose a number of functions including an exponential are nearly equally good fits to the observations explaining 75 of the variance fits to an exponentially decaying function can be extrapolated to give an estimate of 154 38 sv for the full depth transport

eddy induced modulation of turbulent dissipation over rough topography in the southern ocean abstractmesoscale eddies are universal features of the ocean circulation yet the processes by which their energy is dissipated remain poorly understood one hypothesis argues that the interaction of strong geostrophic flows with rough bottom topography effects an energy transfer between eddies and internal waves with the breaking of these waves causing locally elevated dissipation focused near the sea floor this study uses hydrographic and velocity data from a 1 yr mooring cluster deployment in the southern ocean to test this hypothesis the moorings were located over a small 10 km topographic obstacle to the east of drake passage in a region of high eddy kinetic energy and one was equipped with an adcp at 2800 m depth from which internal wave shear variance and dissipation rates were calculated examination of the adcp time series revealed a predominance of upward propagating internal wave energy and a significant correlation r 0 45 between shear variance levels and subinertial near bottom cur

estimates of eddy heat flux crossing the antarctic circumpolar current from observations in drake passage abstractthe 4 yr measurements by current and pressure recording inverted echo sounders in drake passage produced statistically stable eddy heat flux estimates horizontal currents in the antarctic circumpolar current acc turn with depth when a depth independent geostrophic current crosses the upper baroclinic zone the dynamically important divergent component of eddy heat flux is calculated whereas full eddy heat fluxes differ greatly in magnitude and direction at neighboring locations within the local dynamics array lda the divergent eddy heat fluxes are poleward almost everywhere case studies illustrate baroclinic instability events that cause meanders to grow rapidly in the southern passage where eddy variability is weak heat fluxes are weak and not statistically significant vertical profiles of heat flux are surface intensified with 50 above 1000 m and uniformly distributed with depth below summing poleward transient eddy heat transport across the lda of 0 010 0 005 pw with the stat

lateral flow of hot plume material ponded at sublithospheric depths local buoyancy forces drive the flow of ponded plume material along the base of the lithosphere this processes complicates the geological effects of plumes beyond those of simple passage of plates over point heat sources variations in the depth to the base of the lithosphere significantly affect lateral flow the flow rate is modified to a moderate extent by locally thin lithosphere beneath midplate hotspots such as hawaii the effect is relevant when numerical models are rescaled to better fit the observed uplift and the observed distance from the hotspot to the nose of the swell variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes effectively forming an upside down drainage pattern plume material flows upslope toward ridge axes from off axis plumes and along ridge axes from on axis plumes at sufficiently low spreading rates thinner lithosphere exists above plume material than above normal asthenosphere the edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material pressure release melt is associated with the upslope flow of plume material it is expected to cause volcanism away from the plume and to sometimes supply melt depleted plume material to ridge axes that are upslope from the region of voluminous melting

evolution of the carlsberg ridge between 60 and 45 ma ridge propagation spreading asymmetry and the deccan reunion hotspot despite the availability of numerous geophysical data no convincing magnetic anomaly identification and structural map of a major part of the arabian and eastern somali basins have been published to date i propose a new interpretation of the available magnetic data based on the identification of tiny wiggles i e singular second order patterns of the magnetic signal only one fracture zone and a set of propagating rifts existed between the owen fracture zone and the chagos laccadive ridge between anomalies 26 and 20 at anomalies 26 25 at least two eastward propagating rifts are identified while at anomalies 24 20 seven westward propagating rifts are confidently recognized the major consequence of this systematic ridge propagation is a tremendous regional asymmetry between anomalies 26 and 25 about 65 of the crust formed at the carlsberg ridge was accreted to the african plate while at anomalies 24 20 more than 75 benefited to the indian plate i suggest that these asymmetries result from the relative position of the carlsberg ridge and the nearby deccan reunion hotspot the ridge tending to remain near the hotspot

sea floor spreading in the presence of the iceland plume the structure of the reykjanes ridge at 61 40 n abstract we present results from a combined normal incidence and wide angle seismic survey of the reykjanes ridge at 61 40 n in the north atlantic the seismic data show that the crust is 8 4 10 0 km thick on the ridge axis we attribute this thicker than normal oceanic crust to a high degree of melting caused by hot mantle from the iceland plume beneath the speading centre layer 2a of the oceanic crust is significantly thicker at 0 ma than in crust of age 4 ma our results combined with other seismic data from the north atlantic show that the crustal thickness decreases away from the plume centre to normal oceanic crustal thickness at a distance of approximately 1000 km

reunion hotspot magma chemistry over the past 65 m y results from leg 115 of the ocean drilling program leg 115 of the ocean drilling program recovered basalts from four locations along the hotspot track that leads from the deccan flood basalts in india to reunion island in the western indian ocean sites 706 707 713 and 715 the drilled basalts range in age from 35 ma site 706 to 64 ma site 707 and including the deccan basalts 66 to 68 ma mauritius island 0 2 to 8 ma and reunion island 0 to 2 ma seven sites are provided for sampling the volcanic record of the 5 000 km long hotspot track chemical and age comparisons indicate that site 707 lavas correlate with basalt units near the top of the deccan flood basalt sequence the lavas of site 715 55 to 60 ma are most similar to the islands of mauritius and reunion site 713 basalts 48 ma are similar to the earliest lavas of the deccan province and site 706 basalts are intermediate in chemistry between those of central indian spreading ridge basalts and reunion differences in lava compositions along the hotspot track can be related to variable mixing of plume and asthenospheric mantle depending on the changing position of spreading ridge segments and the hotspot during the opening of the indian more ocean alternatively time dependent changes in the composition of hotspot melts may be due to a decrease in partial melting of a heterogeneous plume or to intrinsic changes in the composition of material supplied by the plume less

gulf stream path and thermocline structure near 74 w and 68 w the synoptic ocean prediction synop experiment had the goal of providing a physical understanding of energetic mesoscale eddy processes in the gulf stream in the synop inlet array off cape hatteras and in the central array near 68 w modred observations were collected from october 1987 through august 1990 the inlet array measured the surface path and bottom currents where the gulf stream leaves the continental margin to enter the deep water regime small amplitude propagating and growing meanders characterized the variability there the central array measured velocity and temperature as a proxy for density at four levels in the water column as well as the upper and deep level streamfunctions all with mesoscale resolution near 70 w the path envelope exhibited a relative node confined within a 40 km band 55 of the time near 68 w the path envelope was over 3 times as wide due to several elongated steep meander troughs and relatively steep meander crests the crests typically propagated downstream without much growth the troughs often stalled near 68 w steepened and persisted for one to several months two cases evolved into s shaped paths and subsequently formed rings even the time averaged fields showed a small trough in the mean path and thermocline structure whereas meanders of 20 to 60 day periods had similar spectral levels throughout 70 67 w meanders with long periods 85 day accounted for the local minimum in variance at 70 w bottom pressure and velocity observations revealed repeated periods of intense swirl speeds 0 30 m s 1 abyssal eddies the time averaged deep currents exhibited a mean cyclone centered 30 km offshore and downstream of the upper layer mean trough the cross stream slope of the thermocline steepened linearly with path curvature consistent with gradient wind balance structures are illustrated in the mapped fields consistent with baroclinic instability wherein troughs steepen and rings form

transport and variability of the antarctic circumpolar current in drake passage the baroclinic transport of the antarctic circumpolar current acc above 3000 m through drake passage is 107 3 10 4 sv and has been steady between 1975 and 2000 for six hydrographic sections 1993 2000 along the world ocean circulation experiment woce line sr1b the baroclinic transport relative to the deepest common level is 136 7 7 8 sv the acc transport is carried in two jets the subantarctic front 53 10 sv and the polar front pf 57 5 5 7 sv southward of the acc the southern antarctic circumpolar current transports 9 3 2 4 sv we observe the pf at two latitudes separated by 90 km this bimodal distribution is related to changes in the circulation and properties of antarctic bottom water three realizations of the instantaneous velocity field were obtained with lowered adcps from these observations we obtain near bottom reference velocities for transport calculations net transport due to these reference velocities ranges from 28 to 43 sv consistent with previous estimates of variability the transport in density layers shows systematic variations due to seasonal heating in near surface layers volume transport weighted mean temperatures vary by 0 40 c from spring to summer a seasonal variation in heat flux of about 0 22 pw finally we review a series of papers from the international southern ocean studies program the average yearlong absolute transport is 134 sv and the standard deviation of the average is 11 2 sv the error of the average transport is 15 to 27 sv we emphasize that baroclinic variability is an important contribution to net variability in the acc

bottom pressure torque and the vorticity balance from observations in drake passage the vorticity balance of the antarctic circumpolar current in drake passage is examined using 4 years of observations from current and pressure recording inverted echo sounders the time varying vorticity planetary and relative vorticity advection and bottom pressure torque are calculated in a two dimensional array in the eddy rich polar frontal zone pfz bottom pressure torque is also estimated at sites across drake passage mean and eddy nonlinear relative vorticity advection terms dominate over linear advection in the local 50 km scale vorticity budget in the pfz and are balanced to first order by the divergence of horizontal velocity most of this divergence comes from the ageostrophic gradient flow which also provides a second order adjustment to the geostrophic relative vorticity advection bottom pressure torque is approximately one third the size of the local depth integrated divergence although the cdrake velocity fields exhibit significant turning with depth throughout drake passage even in the mean surface vorticity advection provides a reasonable representation of the depth integrated vorticity balance observed near bottom currents are strongly topographically steered and bottom pressure torques grow large where strong near bottom flows cross steep topography at small angles upslope flow over the northern continental slope dominates the bottom pressure torque in cdrake and the mean across this drake passage transect 3 to 4 10 9 m s 2 exceeds the mean wind stress curl by a factor of 15 20

on the nonequivalent barotropic structure of the antarctic circumpolar current an observational perspective we examine the vertical structure of the horizontal flow and diagnose vertical velocities in the antarctic circumpolar current acc near the kerguelen plateau using em apex profiling floats eight floats measured horizontal velocity temperature and salinity profiles to 1600 dbar with a vertical spacing of 3 5 dbar four times per day over a period of approximately 3 months horizontal velocity profiles show a complex vertical structure with strong rotation of the velocity vector through the water column the distribution of rotation angles from 1247 profiles is approximately gaussian and rotations of either sign are equally likely forty percent of profiles with speed greater than 5 cm s 1 have a depth integrated rotation of less than 15 degrees over 1300 dbar while the other 60 demonstrate significantly stronger rotation consequently most profiles do not conform to the equivalent barotropic model deep flow parallel and proportional to the surface flow used in simplified dynamical models and in gravest empirical mode climatologies of the acc nevertheless since we find the mean rotation to be zero an equivalent barotropic assumption is valid to first order vertical velocities inferred using conservation of mass and a gradient wind balance in natural coordinates have magnitudes on the order of 100 m day we find robust patterns of upwelling and downwelling phase locked to meanders in the flow as found in earlier studies with the advent of high resolution observations such as those presented here and high resolution models we can advance to a more complete understanding of the rich variability in acc structure that is neglected in the equivalent barotropic model

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

three dimensional density model of the nazca plate and the andean continental margin 1 we forward modeled the bouguer anomaly in a region encompassing the pacific ocean 85 w and the andean margin 60 w between northern peru 5 s and patagonia 45 s the three dimensional density model that reproduces the gravity field is a continental scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic nazca plate subducted slab and continental margin with a minimum number of bodies we predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressure temperature conditions appropriate for the andean setting a database of independent geophysical information constrains the geometry of the top of the subducted slab locally the moho of the oceanic and continental crusts and indirectly the lithosphere asthenosphere boundary underneath the continental plate other boundaries notably the intracrustal density discontinuity separating upper from lower crust below the continent were not constrained and their geometry is the result of fitting the observed and calculated bouguer anomaly during forward modeling this contribution presents the model to the andean geoscientific community and contains some tools like a sensitivity analysis that helps potential users of the model to interpret its results we describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena e g modification of oceanic plate structure by hot spots shape of the subducted slab thermal structure of the continental lithosphere compensation mechanism and formation of orogenic relieve causes of andean segmentation

synoptic maps of temperature and velocity within the subantarctic front south of australia 1 from april 1995 to march 1997 a 450 km array of inverted echo sounders horizontal electric field recorders and current meters measured the horizontal and vertical structure of the current temperature and salinity fields associated with the subantarctic front saf south of australia synoptic maps of the temperature and velocity fields often show the saf divided into two zonally separated jets which are also found in the mean these jets were in close proximity yet directed toward different azimuths the daily maps also show that the saf often flowed as a single strong jet during which times it underwent vigorous meandering with crest to trough meridional distances 250 km these meanders stalled within the array and contorted to form s shaped paths in one case a cold core ring was formed propagating meanders have periods of 20 70 days wavelengths of 240 420 km and phase speeds of 12 6 km d 1 deep currents reveal strong cyclones and anticyclones propagating primarily eastward through the array beneath the meandering jet thus at times the deep flow is aligned with the upper flow whereas at other times significant barotropic flows cross the upper front occasionally the northern polar front was also found within the array the observed variability illustrates how the interaction of a meandering current with transient features such as meanders and eddies makes the identification of a front difficult when a single isotherm or isopycnal is utilized as the definition

origin and compensation of chagos laccadive ridge indian ocean from admittance analysis of gravity and bathymetry data abstract the chagos laccadive ridge clr is a prominent aseismic volcanic ridge in the northern indian ocean the ridge together with the southern mascarene plateau smp to which it is genetically related is considered as a volcanic trace of the reunion hotspot we have examined the isostatic compensation of the clr through transfer function analysis of gravity and bathymetry data along seven profiles the analysis suggests that the clr is compensated locally with an airy crustal thickness t c of 20 km the rather low elastic plate thickness t e of about 4 km implies that the volcanism of the ridge took place very near a spreading centre the proximity of the chagos fracture zone indicates that the emplacement was probably near a spreading centre transform junction

cyclogenesis in the deep ocean beneath the gulf stream 2 dynamics strong cyclones in the deep ocean beneath the gulf stream have been observed during the period june 13 1988 to august 7 1990 near 68 w 37 n in data from the synoptic ocean prediction synop experiment these cyclones developed in association with the evolution of large amplitude quasi stationary meander troughs in the gulf stream it is likely that baroclinic instability is responsible for cyclone spin up the dynamical analysis of these cyclones indicates the process is analogous with atmospheric cyclogenesis from the perspectives of divergence and conservation of potential vorticity but not in terms of the density field evolution large positive vertical velocities in the thermocline over developing low pressure centers at 3500 m are consistent with convergence at depth and divergence in the upper ocean and with stretching of the lower water column and shortening of the upper water column the stretching of the lower water column accounts for the generation of positive relative vorticity there however the evolution of the density field in the oceanic case does not resemble the atmospheric case in the atmosphere density field adjustments in the air column above the low pressure center at the earth s surface are in the correct sense to account for decreasing pressure there in the ocean density field adjustments in the water column fail to account for the developing low pressure centers so sea surface height depressions must be responsible these depressions must have an approximate magnitude of 0 5 m depth over a 250 km horizontal extent the cyclone s diameter

variation of effective elastic thickness and melt production along the deccan reunion hotspot track we estimate the effective elastic thickness te along the deccan reunion hotspot track using admittance analysis of seafloor topography and the free air gravity field both corrected for the thermal effects of a cooling lithosphere our results reveal that the volcanic edifices saya de malha bank chagos maldives laccadives ridge formed in the first 30 myr after the deccan volcanism not vert similar 65 myr on lithosphere with te values of 4 2 km while the younger volcanic edifices on the african plate reunion mauritius nazareth bank were emplaced on lithosphere with te values of 17 9 km these estimates suggest that the hotspot volcanism occurred on juvenile lithosphere in the first 30 myr implying that the mid ocean ridge remained near the hotspot for not vert similar 30 myr in contrast in the last 30 myr volcanism occurred on aged lithosphere in an intraplate setting which might indicate that the mid ocean ridge migrated rapidly to the north after the african plate moved over the hotspot this conclusion of a rapid shift from plume influenced mid ocean ridge mor volcanism to intraplate plume volcanism is supported by geochemical major and trace element interpretations of data from ocean drilling program odp leg 115 an estimate of the melt production rate shows a striking increase in the small te region relative to the large te region of the hotspot track which suggest a strong interrelation between te and melt production however there is also variation of melt emplacement rates within the region of low te that may be due to unknown changes in the rates of plate motions or somewhat episodic melt production

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

crustal structure of the cocos ridge northeast of cocos island panama basin 1 the submarine cocos ridge in the northwestern panama basin pacific ocean is generally interpreted as the trace of the galapagos hotspot a 278 km long seismic wide angle transect was carried out across a comparatively narrow ridge segment 150 km northeast of cocos island the results indicate a huge thickened crust where crustal thickening is mainly achieved by the lower crust which makes up 75 of the crustal thickness and is tripled compared to normal oceanic lower crust the velocities are comparable to normal lower crust and suggest no differences to a gabbroic composition the moho deepens from 10 to 18 km depth below the ridge similarities to other hotspot related oceanic ridges and plateaus with massive lower crusts and velocities below 7 4 km s suggest the formation of this cocos ridge segment near or at the plate boundary

investigating the crustal structure of a strike slip step over zone along the great glen fault strike slip step overs and bends are regions of anomalous deformation that may yield clues to the mechanical behavior of the earth s crust experimental reprocessing of a marine deep seismic reflection profile cutting across an 35 km wide right stepping step over developed on the great glen fault system north of the inner moray firth basin east of northwest highlands scotland reveals a restricted zone of prominent reflections and diffractions beginning at 7 8 km depth continuing as deep as the interpreted moho discontinuity at 26 27 km geological interpretation of this zone suggests a concentration of possible diffractors marking sharp structural disruption underlain by a 12 km thick layer of subhorizontal and moderately dipping reflectors that dominates the lower crust in the step over region the known kinematic history of the great glen fault system together with the observed direction of the step over implies that a zone of contraction would have formed within the step over during early caledonian sinistral strike slip the seismic reflection structure can be interpreted as developing by deformation associated with contraction and possible block rotation between the two fault segments such that the diffractive zone represents steep structure in a near vertical zone of strike slip which passes deeper into a layer of low angle dipping thrusts or shear zones the results of this study are consistent with a concept of mechanical detachment in the middle crust as documented for areas of contraction along active strike slip faults

plume transform interactions at ultra slow spreading ridges implications for the southwest indian ridge 1 we explore a potentially important variable in controlling ridge hot spot interaction the effect of transform offsets in limiting along axis flow of plume material we focus on the southwest indian ridge swir where the transform damming effect is likely to be pronounced because of both long offset lengths and large contrasts in lithospheric thickness across the transform faults due to the ultra slow spreading rate we investigate the degree to which transform faults affect axial asthenospheric flow by performing a series of three dimensional 3 d numerical experiments with simplified channel flow geometry and extrapolating their results to the swir 3 d mantle viscosity structure for a ridge transform ridge system is determined based on temperature and pressure dependent viscosity laws we consider six transform lengths spanning 0 to 250 km in increments of 50 km we then calculate the 3 d viscous flow in response to an along axis pressure gradient corresponding to a ridge centered hot spot modeling results predict that transform faults affect along axis asthenospheric flow in two important ways first transforms reduce along axis flux the longer the transform offset the greater the reduction in across transform flux relative to the zero offset case second transforms deflect shallow asthenospheric along axis flow the predicted transform damming effect is most pronounced for a viscosity structure that is strictly pressure and temperature dependent flux reduction effects could be less significant for viscosity laws that additionally consider dehydration melting and change in deformation mechanism this model predicts that the waist width of an on axis plume is dependent not only on such previously explored factors as buoyancy and spreading rate but also on the geometry of ridge segmentation along the swir axial flow driven by the marion plume is likely curtailed by the long offset andrew bain and discovery ii fracture zones severely limiting its lateral extent

improving estimates of the antarctic circumpolar current streamlines in drake passage abstract accurately resolving the mean antarctic circumpolar current acc is essential for determining southern ocean eddy fluxes that are important to the global meridional overturning circulation previous estimates of the mean acc have been limited by the paucity of southern ocean observations a new estimate of the mean surface acc in drake passage is presented that combines sea surface height anomalies measured by satellite altimetry with a recent dataset of repeat high resolution acoustic doppler current profiler observations a mean streamfunction surface height field objectively mapped from the mean currents is used to validate two recent dynamic height climatologies the new streamfunction has narrower and stronger acc fronts separated by quiescent zones of much weaker flow thereby improving on the resolution of acc fronts observed in the other climatologies distinct streamlines can be associated with particular acc fronts and tracked in time dependent maps of dynamic height this analysis

an eddy permitting southern ocean state estimate abstract an eddy permitting general circulation model of the southern ocean is fit by constrained least squares to a large observational dataset during 2005 06 data used include argo float profiles ctd synoptic sections southern elephant seals as oceanographic samplers seaos instrument mounted seal profiles xbts altimetric observations envisat geosat jason 1 and ocean topography experiment topex poseidon and infrared and microwave radiometer observed sea surface temperature an adjoint model is used to determine descent directions in minimizing a misfit function each of whose elements has been weighted by an estimate of the observational plus model error the model is brought into near agreement with the data by adjusting its control vector here consisting of initial and meteorological boundary conditions although total consistency has not yet been achieved the existing solution is in good agreement with the great majority of the 2005 and 2006 southern ocean observations and better repr