major pathways of atlantic water in the northern north atlantic and nordic seas toward arctic 1 the major pathways of near surface atlantic water in the northern north atlantic and nordic seas are identified as current speeds above 30 cm s using 1014 lagrangian drifters combined with previously published hydrography the inflow over the scotland greenland ridge and establishment of the two branch norwegian atlantic current nwac are described in light of the circulation in the northern north atlantic the western branch of the nwac appears as a jet in the polar front topographically guided from the iceland faroe front through the nordic seas toward fram strait the eastern branch starts as a shelf edge current above the irish scottish continental shelf and after passing through the faroe shetland channel it continues northward along the norwegian shelf edge toward the arctic with a branch bifurcating into the barents sea the nwac appears to maintain its two branch structure throughout the nordic seas with the atlantic water confined to a 200 600 km wide wedge

the geomorphology of the southwest guinea margin tectonic volcanic mass movement and bottom current influences abstract slope morphology controls were examined in two topographically different areas on the southern transform margin of the guinea plateau sectors g1 and g2 tectonic control is manifested by steep faulted slopes in both study areas and by basement warping in sector g1 and magmatic emplacements in sector g2 sediment deposition and the formation of bedforms such as sediment waves on the margin are influenced by topographically intensified bottom currents within the intermediate deep water mass the north atlantic deep water nadw as well as in the shallower equatorial surface water esw antarctic intermediate water aaiw at the uppermost slope plateau edge level thus unconsolidated sediments are eroded from steep slopes the upper and lower slopes canyon walls and the slopes of the volcanic cones and concentrated into slope parallel depocentres in sector g1 and ponded in moats between the volcanic cones in sector g2 surficial sediment waves which are apparently oriented at an angle to the slope range from 0 5 to 2 km in wavelength and up to 100 m in height mass movement of the sediment cover is significant on the lower slopes of both sectors and is the result of slope steepness and instability caused by the continuing tectonic activity

s velocity variations beneath north america 1 we investigate s velocity variation in the upper mantle beneath north american to better understand the effects of data heterogeneity model parameterization and regularization to this end we analyzed and fit regional s and rayleigh wave trains generated by earthquakes around north america that occurred between the years 2000 through 2006 including waveforms from the transportable array stations of earthscope s usarray these new data were combined with constraints used for the 3 d s velocity model na04 in order to create a new model na07 another model was created from a suite of good fit models to provide a useful guide for model velocities and uncertainties by estimating ranges of probable velocity variations throughout the upper mantle we find that the southern and eastern edges of the north american craton appear to be defined by paleozoic orogens rather than proterozoic ones on average the archean portion of the craton is 200 km thick while the paleozoic part averages 175 km thick with an 80 m s lower s velocity the horizontal gradients in velocity are over 1 0 100 km at the western margin of the craton 0 5 100 km in the south and 1 100 km at the eastern margin

mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

direct measurement of the deep circulation within the brazil basin abstract in support of the deep basin experiment part of the world ocean circulation experiment a large number of neutrally buoyant floats were released within the brazil basin during the 1990s in an attempt to measure directly the circulation in the deep ocean interior three levels corresponding to the three major subthermocline water masses were selected and results from the deeper two north atlantic deep water nadw and antarctic bottom water aabw are described at this writing processing of acquired tracking data is incomplete hence this paper reports on the progress of the observational program and gives our initial conclusions it appears that the flow in the deep brazil basin is unlike previous conjectures in which the circulation patterns can be characterized as being primarily meridional both along the western boundary and in the interior the existence of a deep western boundary current dwbc is quite clear in the float data at the nadw level but less prominent in the aabw and the interior flow is dominantly zonal with unexpectedly small meridional space scales integral time scales are long of order 20 30 days and eddy kinetic energy levels are low of order 1 cm2 s2 in spite of the low energy levels a surprising number of our floats became caught up in vortices a line of seamounts extending offshore near 20 s known as the vitoria trindade seamounts interrupts the dwbcs and is the location for eddy formation and apparent flow away from the boundary into the interior although it has been speculated that this could feed a narrow zonal current of nadw the namib col current our float trajectories suggest a return to the western boundary rather than a continuation to the east

atlantic meridional overturning circulation during the last glacial maximum the circulation of the deep atlantic ocean during the height of the last ice age appears to have been quite different from today we review observations implying that atlantic meridional overturning circulation during the last glacial maximum was neither extremely sluggish nor an enhanced version of present day circulation the distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the atlantic only slightly greater than today however evidence from multiple water mass tracers supports a different distribution of deep water properties including density which is dynamically linked to circulation

toroidal mantle flow through the western u s slab window the circular pattern of anisotropic fast axis orientations of split sks arrivals observed in the western u s cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global scale plate motion a plume origin for this pattern accounts more successfully for the anisotropy field but little evidence exists for an active plume beneath central nevada we suggest that mantle flow around the edge of the sinking gorda juan de fuca slab is responsible for creating the observed anisotropy seismic images and kinematic reconstructions of gorda juan de fuca plate subduction have the southern edge of this plate extending from the mendocino triple junction to beneath central nevada and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab consistent with the observed pattern of anisotropy this flow may enhance uplift extension and magmatism of the northern basin and range while inhibiting extension of the southern basin and range

secondary flow deflection in the lee of transverse dunes with implications for dune morphodynamics and migration measurements of lee side airflow response from an extensive array of meteorological instruments combined with smoke and flow streamer visualization is used to examine the development and morphodynamic significance of the lee side separation vortex over closely spaced transverse dune ridges a differential deflection mechanism is presented that explains the three dimensional pattern of lee side airflow structure for a variety of incident flow angles these flow patterns produce reversed along dune and deflected surface flow vectors in the lee that are inferred to result in net lateral diversion of sand transport over one dune wavelength for incident angles as small as 10 from crest transverse i e 80 from the crest line this lateral displacement increases markedly with incident flow angle when expressed as the absolute value of the total deflection in degrees reversed and multi directional flow occurs for incident angles between 90 and 50 these results document the three dimensional nature of flow and sand transport over transverse dunes and provide empirical evidence for an oblique migration model copyright 2013 john wiley sons ltd

reversed sediment wave migration in the irish sea nw europe a reappraisal of the validity of geometry based predictive modelling and assumptions abstract the observation and prediction of the mobility of the seabed are in high demand with rapidly expanding offshore engineering developments and the increasing urgency to start mitigation against changing coastlines from repeatedly surveyed sediment wave fields in the irish sea average sediment wave migrations have been mapped from which broad scale net sediment transport directions can be derived the proximity of bathymetric highs and lows seems to redirect sediment migration pathways and increases migration rates by a factor 5 on average via current deflection and amplification a few hundred sediment waves were analysed geometrically in this study and compared to their migration behaviour revealing a lack of statistical correlation sediment wave lee slope directions asymmetry indices and other geometrical parameters appear poorly indicative of their degree of mobility in the irish sea a considerable amount of sediment waves counter intuitively migrate in the direction of their gentle stoss slope i e opposite to the expected sense of migration even when considering the limitations of the predominantly 2 dimensional sampling method these results fundamentally challenge established assumptions on the indicative meaning of sediment wave geometry they question the validity and robustness of sediment wave migratory predictions based on single geometrical readings that fail to represent the complexity of local hydrodynamics and partial sediment wave modifications until predictive tools for sediment wave migration are developed that can pass the test in all aquatic environments repeat surveying with sonar systems provides the only reliable way to record true natural variability

a note on the transport of the brazil current abstract an xbt survey and hydrographic sections between 19 and 25 s along the coast of brazil in april of 1982 were used to locate the brazil current both north and south of the seamount chain at 20 30 s the current appears to flow through the passage between the most inshore of the banks and not to the east it is a continuous feature and to the extent resolved by these measurements is not characterized by meanders and eddies at these latitudes transport relative to 500 m 1000 m was approximately 3 8 106 m3 s 1 6 8 106 m3 s 1 and did not appear to increase downstream

antarctic circumpolar deep water a quaternary paleoflow record from the northern scotia sea south atlantic ocean in the northern scotia sea the main pathway of circumpolar deep water cpdw flows north to pass through a deep gap in the north scotia ridge before turning east into the falkland trough a sediment drift has developed on the seabed since the early middle miocene coincident with the opening of drake passage and the inception of deep water flow seismic and acoustic surveys show that the drift covers an area of 10 500 km 2 and forms a broadly asymmetrical mound up to 800 m thick there is a zone of sediment thinning along the northwestern margin the result of accentuated cpdw flow around rough ocean floor topography small debris flows originating around the margins of the drift suggest localized instability and high sediment supply four cores 3 9 m long have been recovered from the crest and margins of the drift in water depths of 3900 4300 m biostratigraphy and chemostratigraphy reveal that the longest core extends down to oxygen isotope stage 10 approx 370 ka the sediments are predominantly fine grained contourites and diatom rich hemipelagites capped by sandy silty contourites rich in the planktonic foraminifer neogloboquadrina pachyderma grain size analysis of the fine fraction finer than 4 phi 63 mm combined with radiocarbon ams dating and magnetic susceptibility provide an indication of relative cpdw strength over the last 18 ka shortly after the last glacial maximum lgm at approximately 17 ka silt modes fluctuated from 5 5 phi to up to 6 25 phi this increased current winnowing is indicative of an unstable cpdw with stormier glacial benthic conditions producing sporadic high energy currents across the drift crest and flanks at approximately 12 280 ka an increase in sediment sorting is noted indicative of a strong flow of cpdw over the drift crest suggesting an unstable and fluctuating deep water flow during deglaciation and into the holocene at approximately 10 ka cpdw flow stabilized becoming less vigorous across the drift crest and flanks with silt modes from 6 phi to 5 5 phi accompanied by increased sorting of the sediments the gross average sedimentation rate from the crest of the drift is 11 2 cm ky compared to 2 3 cm ky on the southeastern flank the unsteadiness of cpdw during glacials compared to interglacial periods may be the result of stronger wind forcing and a northward shift in the polar front older cpdw flow records from the cores suggest variable and cyclic bottom current flow corresponding to glacial interglacial episodes modern cpdw flow across the crest of the drift averages 11 6 cm s super 1 but with intermittent benthic storm activity resuspending the fines

description of a contourite depositional system on the demerara plateau results from geophysical data and sediment cores the demerara plateau belonging to the passive transform margin of french guiana was investigated during the iguanes cruise in 2013 the objectives of the cruise were to explore the poorly known surficial sedimentary column overlying thick mass transport deposits as well as to understand the factors controlling recent sedimentation the presence of numerous bedforms at the seafloor was observed thanks to newly acquired iguanes bathymetric high resolution seismic and chirp data while sediment cores allowed the characterization of the deposits covering the mass transport deposits modern oceanographic conditions were determined in situ using mooring monitoring over a 10 month period our data indicate the presence of a contourite depositional system along the demerara plateau most likely related to bottom current activity of the north atlantic deep water nadw however at the regional scale large longitudinal waves parallel the nadw flow their shape and orientation seem to be inherited from interactions between bottom currents and paleomorphologies expressed at the top of mass transport deposits their evolution is possibly driven by the intensity of bottom current activity that might have changed over time overall this work presents an integrated approach combining seismic and sedimentological evidence to study the processes at the origin of contourite formation in the demerara plateau other regional factors such as local slope values and slope instability also control sedimentation

adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

evidence in hydrography and density fine structure for enhanced vertical mixing over the mid atlantic ridge in the western atlantic received 24 august 2001 revised 22 february 2002 accepted 25 march 2002 published 15 october 2002 1 anomalous conditions exist in the salinity oxygen and nutrient fields over the western flank of the northern hemisphere mid atlantic ridge we examine possible advective sources for this anomaly but determine that vertical mixing is the most likely cause we proceed to use knowledge gained from the brazil basin tracer release experiment in the south atlantic where microstructure and fine structure were obtained to explore the intensity spatial distribution and mechanisms of mixing in the deep ocean to interpret density fine structure from common conductivity temperature depth data in the north atlantic these data support the hypothesis that the anomalous hydrographic conditions are associated with enhanced levels of vertical mixing the inferred levels of vertical diffusivity over the northern hemisphere mid atlantic ridge are as high as in the south atlantic 1 10 10 4 m 2 s index terms 4532 oceanography physical general circulation 4536 oceanography physical hydrography 4568 oceanography physical turbulence diffusion and mixing processes keywords diapycnal mixing fine structure recirculation mid atlantic ridge mar hydrography silicate

correlated millennial scale changes in surface hydrography and terrigenous sediment yield inferred from last glacial marine deposits off northeastern brazil abstract the stable isotope composition of planktonic foraminifera correlates with evidence for pulses of terrigenous sediment in a sediment core from the upper continental slope off northeastern brazil stable oxygen isotope records of the planktonic foraminiferal speciesglobigerinoides sacculiferandglobigerinoides ruber pink reveal sub milankovitch changes in sea surface hydrography during the last 85 000 yr warming of the surface water coincided with terrigenous sedimentation pulses that are inferred from high xrf intensities of ti and fe and which suggest humid conditions in northeast brazil these tropical signals correlate with climatic oscillations recorded in greenland ice cores dansgaard oeschger cycles and in sediment cores from the north atlantic heinrich events trade winds may have caused changes in the north brazil current that altered heat and salt flux into the north atlantic thus affecting the growth and decay of the large glacial ice sheets

tectonic and geologic evolution of the espanola basin rio grande rift structure rate of extension and relation to the state of stress in the western united states abstract the espanola basin of the rio grande rift began as a broad crustal downwarp in latest oligocene time most of the basin is 2 3 km deep but localized faulting allowed accumulation of up to 5 km of sedimentary fill in a central sub basin the localized early faulting ended before filling of the central espanola basin was completed about 10 m y ago movement on faults that define the present western margin of the espanola basin began 10 m y ago jemez mountain volcanism in the western espanola basin also began at about this same time west tilting of up to 30 occurred due to movement along pervasive n trending intrabasin faults about 7 5 m y ago in conjunction with continued movement along the western border faults volcanism continued after this tilting forming many of the large volcanic constructs of the jemez mountains regional uplift of the entire northern rio grande rift began 7 m y ago movement on the pajarito fault zone began about 5 m y ago and continues to the present this fault zone defines the western margin of the velarde graben a narrow central sub basin where recent movement has been concentrated some volcanism also has occurred within the southern velarde graben total extension across the espanola basin since 26 m y ago is estimated to have been 5 5 km roughly 10 or between 3 5 and 8 km assuming high angle planar faulting the 0 2 mm yr averaged long term rate of extension has been separated into three periods of activity 1 1 0 14 mm yr from 26 to 10 m y ago 2 2 0 5 mm yr from 10 to 5 m y ago 3 3 0 14 mm yr from 5 m y ago to present a change in least principal stress direction from wsw ene to wnw ese that occurred throughout the western united states about 10 m y ago coincides with a roughly 3 5 times increase in the rate of extension preferential development and movement of n to ne trending normal faults and a few degrees of clockwise rotation of rocks in the western espanola basin similar to the espanola basin initial basins of the southern rio grande rift were broad downwarps and rifting was greatly accelerated after 10 m y ago accelerated uplift of the northern rio grande rift also occurred at about this time indicating that activity in the entire rio grande rift was modulated by this change in extension direction 10 m y ago that appears related to pacific north american plate interactions this modulation coupled with major faulting 10 m y ago preceding uplift 7 m y ago in the espanola basin suggest a passive rifting process for the rio grande rift whereby stresses due to plate interactions elsewhere cause faulting in the lithosphere which leads to the development of a passive asthenospheric uplift furthermore the roughly 20 m y pre uplift history of sediment accumulation in basins of the central and southern rift and the inherited character trend and geometry of the rio grande rift as a whole are also more consistent with a passive rifting process

vertical mantle flow associated with a lithospheric drip beneath the great basin rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional scale downwelling of lithospheric mantle however because lithospheric drips sinking plumes of cold and dense lithosphere are relatively small and transient features direct evidence of their existence has been difficult to obtain moreover the significant vertical mantle flow that should be associated with such structures has not been detected here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the great basin region of the western united states the feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1 and a localized temperature increase of 10 the drip tilts to the northeast opposite to the motion of the north american plate in the hotspot reference frame and thereby indicates northeast directed regional mantle flow seismic anisotropy data for the great basin region of the western united states coupled with tomographic images help delineate a northeast dipping lithospheric drip numerical experiments suggest that the drip could have formed owing to gravitational instability triggered by a density increase of about 1 and a temperature increase of about 10

the interplay between turbidity and contour current processes on the columbia channel fan drift southern brazil basin abstract the distribution of sedimentary processes and associated deposits on the columbia channel fan drift system southern brazil basin were investigated on the basis of 3 5 khz echosounding profiles and core lithology this system is composed of a sediment levee elongated downslope with a w e then sw ne trend bounded to the south by the columbia channel and to the north by the vitoria trindade seamounts turbidites are widespread on the shallowest part of the system 1 thick turbidite sequences composed of quartz rich sandy material originating from the upper continental margin are found in the axis of the channel the turbidity currents do not significantly overflow the northern levee where only sparse and thin silty muddy turbidites occur close to the axis of the channel 2 the major part of the levee shows quartz poor mica and foram rich turbidite material originating from the vitoria trindade seamounts the abundance of this material decreases southwards and eastwards the whole area is swept by the northward flowing antarctic bottom water contour current deposited muds are dominant on the southern flank of the columbia channel free of any turbidite supply as a consequence of the coriolis effect and on the deepest part of the northern levee remote from turbidite source areas however the contour currents interplay with the turbidity currents on the shallowest part of the levee contour current deposited muds are interbedded with silty sandy turbidite sequences and a part of the muddy material has probably been pirated from the finer tails of the turbidity flows the other part being transported from southernmost areas by the bottom currents the interplay of both processes is also responsible for the deposition of silty turbidite beds top truncated by contour currents and overlain by bioturbated muddy contourites

transform marginal plateaus abstract numerous submarine plateaus form highstanding bathymetric highs at continent to ocean transitions due to their proximity to continents they have been frequently labelled marginal plateaus although this term has not been clearly defined or associated with a specific geology or geodynamic process until now these elevations have been interpreted as submerged thinned continental fragments detached from continents basaltic buildups formed by hotspots volcanic margins or oceanic plateaus many of these plateaus formed at transform margins connecting oceanic basins of contrasted ages we propose for the first time to define and review a class of marginal plateaus related to a specific tectonic setting transform marginal plateaus tmps based on a compilation of 20 tmps around the world we show that most of them have a polyphased history and have undergone at least one major volcanic phase our review highlights in particular a hitherto unrecognized close link between hotspots volcanic activity and transform margins we also propose that due to their polyphased history tmps may contain several successive basins and overlooked long lived sedimentary archives we finally highlight that because these tmps were transform plate boundaries perpendicular or oblique to surrounding rifts many of them were close to last contact points during final continental breakup and may have formed land bridges or bathymetric highs between continents therefore we discuss broader scientific issues such as the interest of tmps in recording and studying the onset and variations of oceanic currents or past biodiversity growth bio connectivity and lineage evolution

three dimensional seismic velocity structure of the northwestern united states 1 we present a high resolution three dimensional p wave tomography model for the northwestern united states major features of the model include clear well resolved images of the juan de fuca gorda slab to depths of at least 500 km reduced velocities beneath newberry volcano and north central oregon increased velocities beneath the idaho batholith and a deep slab like eastward dipping feature of increased velocities beneath nevada we find no evidence for a mantle plume in this region and demonstrate that suggested disruption of the juan de fuca slab beneath oregon in the vicinity of newberry volcano is an inversion artifact we interpret the primary features of this new model within the framework of post laramide tectonomagmatism and the continuing tectonic evolution of the cascadia subduction zone a model explaining the regional tectonomagmatic history consistent with the imaged seismic velocity structure is one driven by mantle flow responding to the evolving subduction system

contourite drift molded by deep mediterranean outflow since its inception in early pliocene time the deep mediterranean outflow has eroded and sculptured the southern iberian margin the 50 km long 300 m thick faro drift constructed by this powerful bottom current has been the subject of detailed study sandy silty and muddy contourite facies can be clearly characterized and a distinctive vertical arrangement or sequence of these facies can be related to hydrodynamic fluctuation these results are important for the identification of ancient contourites the reconstruction of paleocirculation patterns and documentation of the effects of quaternary climates on the mediterranean outflow

the mean flow field of the tropical atlantic ocean the mean horizontal sow eld of the tropical atlantic ocean is described between 20in and 20is from observations and literature results for three layers of the upper ocean tropical surface water central water and antarctic intermediate water compared to the subtropical gyres the tropical circulation shows several zonal current and countercurrent bands of smaller meridional and vertical extent the wind driven ekman layer in the upper tens of meters of the ocean masks at some places the sow structure of the tropical surface water layer as is the case for the angola gyre in the eastern tropical south atlantic although there are regions with a strong seasonal cycle of the tropical surface water circulation such as the north equatorial countercurrent large regions of the tropics do not show a signi cant seasonal cycle in the central water layer below the eastward north and south equatorial undercurrents appear imbedded in the westward sowing south equatorial current the antarcic intermediate water layer contains several zonal current bands south of 3in but only weak sow exists north of 3in the sparse available data suggest that the equatorial intermediate current as well as the southern and northern intermediate countercurrents extend zonally across the entire equatorial basin due to the convergence of northern and southern water masses the western tropical atlantic north of the equator is an important site for the mixture of water masses but more work is needed to better understand the role of the various zonal under and countercurrents in cross equatorial water mass transfer 1999 elsevier science ltd all rights reserved

mantle structure beneath the western edge of the colorado plateau 1 teleseismic traveltime data are inverted for mantle vp and vs variations beneath a 1400 km long line of broadband seismometers extending from eastern new mexico to western utah the model spans 600 km beneath the moho with resolution of 50 km inversions show a sharp large magnitude velocity contrast across the colorado plateau great basin transition extending 200 km below the crust also imaged is a fast anomaly 300 to 600 km beneath the nw portion of the array very slow velocities beneath the great basin imply partial melting and or anomalously wet mantle we propose that the sharp contrast in mantle velocities across the western edge of the plateau corresponds to differential lithospheric modification during and following farallon subduction across a boundary defining the western extent of unmodified proterozoic mantle lithosphere the deep fast anomaly corresponds to thickened farallon plate or detached continental lithosphere at transition zone depths

sedimentation resuspension and chemistry of particles in the northwest atlantic abstract sediment traps were deployed to sample vertical fluxes of particulate matter both immediately above and within the nepheloid layer on the continental slope and rise of the western north atlantic the data from the traps are used to estimate the settling flux of particles and to attempt to distinguish between the primary flux settling material from the upper water column and the resuspended flux settling material that has been resuspended from some region of the sea floor from these data and from measurements of the net nepheloid standing crop of particles estimates of the mean residence time for particles resuspended in the nepheloid layer are on the order of weeks to months from 15 to 500 m above bottom horizontal advection and diffusion are more important than vertical mixing in maintaining the upper portion of the nepheloid layer the estimated flux of al to the north atlantic as atmospheric dust is one to two orders of magnitude less than the primary flux of al measured just above the nepheloid layer yet the range of accumulation rates of al measured on the sea floor with radioisotopes is close to the values we define as the primary flux horizontal transport of al rich particles mostly aluminosilicates must therefore be significant even above the nepheloid layer the size of 80 90 of the particles collected in the traps both above and within the nepheloid layer was less than 63 m the mean size of particles trapped in the nepheloid layer was about 22 m and above the nepheloid layer 11 m the particles were probably in large aggregates when they encountered the traps but identifiable fecal pellets are only a small fraction of the material collected in the traps between 6 and 15 of the primary production of particulate organic carbon in the euphotic zone was collected in floating sediment traps at the 100 m depth indicating that most particulate organic carbon is recycled in the surface water only 2 3 of primary production of organic carbon was collected as primary flux 500 m above the sea floor from an intercomparison of the composition of particles falling rapidly collected in traps falling slowly or not at all collected in water bottles and resting on the sea floor from core tops it was found that elements associated with biogenic matter such as ca sr cu and i were carried preferentially by the rapidly falling particles once these particles reached the sea floor the concentration of those elements was decreased through decomposition respiration or dissolution dissolution appears to be rapid near or on the sea floor because radiolarians diatoms and juvenile foraminifera were rare in core samples even though abundant in all traps the dynamic nature of the nepheloid layer makes it possible for particles to be resuspended many times before they are finally buried enabling sediment to be carried long distances from its origin the recycling of particles near the sea floor may also increase dissolution of siliceous and carbonate matter

a giant sandy sediment drift in early silurian telychian and its multiple sedimentological prosess abstract this study describes a regional scale sedimentary feature in the upper yangtze region of china that is identified as a telychian age 438 ma sediment sandy drift that is 500 800 km long 100 150 km wide and 10 20 m thick graptolite zones is applied to recognize telychian sandstone beds and reconstruct an isochronal stratigraphic framework for the study area it is possible to observe numerous traction structures in telychian sandstone beds including ripples cross bedding cross laminations wavy laminations and horizontal laminations and bouma sequence the palaeocurrent direction is towards ssw 200 240 which is measured by sedimentary structures cross bedding cross laminations and ripples in outcrops besides the alignment of elongate particles in thin section and the distribution characteristics of heavy minerals also indicate a ssw palaeocurrent direction this direction is parallel to palaeo coastline which is a characteristic feature of bottom current a multiple sedimentary processs is identified based on analysing facies of each sections contourite dominated sequences are observed in channel related seaway while turbidite sequences are observed in shelf near palaeo coastline based on provenance analysis the telychian drift channel reworked sandy drift is the production of a mixed depositional system first turbidite deposition and then bottom current deposition the turbidite sand on the shelf provides material source for the sandy drift and the deposit is transported from the shelf to the channel parallel to ancient coastline the provenance direction is identified by analyzing the distribution of heavy minerals the unstable heavy minerals content is higher in the turbidites samples near provenance region than the channel reworked sandstone samples far away from provenance region this telychian deposit is a rare example of an ancient channel reworked sandy drift

oceanic density fronts steering bottom current induced sedimentation deduced from a 50 ka contourite drift record and numerical modeling off nw spain abstract how the various bottom near hydrographic and sedimentary processes control the formation of contourite drifts i e of bottom current related confined deep sea depocenters usually remains widely speculative this study uses a transect of six sediment cores and a sediment echosounder profile across a whole contourite system off nw spain to address the sediment dynamics responsible for the depositional pattern this mounded patch type contourite drift 18 km long 20 km wide with a 150 m deep channel moat has formed around an 800 m high structural obstacle deposition on the contourite drift in the past was characterized by alternating calm and high energy bottom flow conditions calm conditions last glacial period 27 17 cal ka bp late holocene times process based numerical modeling demonstrates that pulse like oceanic density fronts traveling within the transition zone of two water masses labrador sea water mediterranean outflow water provide a powerful mechanism for contouritic deposition rather than the core of a water mass itself these gravity driven density fronts lead to local re suspension of sands stored inside the drift s moat and to subsequent upward transport towards the drift s crest here the oceanic density fronts produce additional km scale eddies these migrating eddies provide an efficient mechanism for further widespread sediment re distribution in comparison with paleoceanographic reconstructions a downward migration or expansion of the mediterranean outflow water by about 300 m led most probably to such temporary contouritic sand deposition we finally propose a conceptual model to explain how seafloor obstacles redirect and perturbate bottom currents this model proposes not only a sharp contact between two water masses but also the transition zone between those as an important high energy regime offering oceanic density fronts a travel medium these fronts are strong enough to distribute fine sands across highly pronounced seabed topography on the respective time scale the moat itself seems to act as the main source for those sands making a remote source and a long distance sediment transport unnecessary

distribution morphology and origin of sedimentary furrows in cohesive sediments southampton water patches of sedimentary furrows are developed at several locations in the cohesive estuarine sediments of southampton water water depth 1 12 m these furrows apparently result from short periods of erosion followed by long periods of deposition although all the furrows are similar regularly spaced parallel troughs 0 5 15 m wide aligned with the dominant current furrows in different patches have different characteristics in some areas furrow width is 1 5 1 15 of furrow spacing termed narrow whereas in other areas furrow width is about 1 2 of the spacing termed wide narrow furrows have developed where sediment accumulation rates are greater than 3 6 cm yr 1 wide furrows where accumulation rates are lower cockle shells and other coarse sediments concentrated on the furrow floors and on floors of smaller 2 10 cm wide minifurrows play an important role in furrow formation and evolution as they act to widen the furrows when mobilized during current episodes uniform sedimentation across the profile during slack periods tends to narrow the furrow some of the larger furrows have remained in the same position for 12 years while mini furrows have duration scales of a few months or less well developed furrows are also found in a recently dredged channel bedforms similar to those described here may be preserved in the sedimentary record while no analogues to the larger furrows are presently known minifurrows may be morphologically similar to the gutter casts described from ancient rocks

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

giant mounded drifts in the argentine continental margin origins and global implications for the history of thermohaline circulation abstract partially buried giant drifts are located in the southern most sector of the argentine continental margin generating a bathymetric jump at the base of the slope they are characterised as giant asymmetrical elongated mounded contourite drifts and are described in detail here for the first time this description is mainly based on the bathymetric and multichannel seismic reflection profiles data their origin and evolution in the argentine basin are decoded and their implications in an area crucial for geologic and palaeoceanographic reconstruction between the atlantic and antarctica are discussed the buried giant drifts are divided into two zones by a large seamount the major giant drift 50 km wide 300 km long and having a sedimentary thickness of nearly 1 km is located to the south of this seamount and oriented to the north where its summit outcrops at present seafloor here its asymmetric external shape is characterised by a steep western side and a gently dipping smooth eastern side with internal reflections prograding eastward based on its position morphology and internal characteristics it has been deduced that this giant drift was generated in the argentine basin by the southward branch of a confined large loop of the antarctic bottom deep water from the eocene oligocene boundary coevally with the drake passage opening until the middle miocene however after a major palaeoceanographic change in the mid to late miocene a new oceanographic scenario was established that fossilised these giant drifts these changes produced a new depositional style in the margin resulting in initiation of the present contourite depositional system and morphologic features over the slope extension of north atlantic deep water circulation into the southern hemisphere and deepening of antarctic bottom water circulation in the argentine basin are herein postulated as the factors that controlled said changes these giant drifts show strong seismic evidence of hydrates and free gas that are ripe for energy resources exploration thus demonstrating the economic potential of contourite deposits in deep marine environments

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

radiometric dating of submarine hydrocarbon seeps in the gulf of mexico massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid bathymodiolus sp containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and u series isotopes stable carbon isotopes and 87 sr 86 sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon rich fluids samples from five seepage sites on the northern gulf of mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to 2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives in general paired 14 c and 230 th dates of carbonate buildups consisting of aragonite high mg calcite and dolomite mineralogies are highly discordant the cause of the discordance established on the basis of paired 14 c and 13 c values 14 c 898 to 992 13 c 9 5 to 53 3 for n 27 lies with the impairment of the radiocarbon dates resulting from dilution of the 14 c pool with fossil hydrocarbon derived carbon the validity of the ionium dates based on u rich 2 4 7 6 ppm samples is demonstrated by the concordance between 234 u 238 u and 230 th 234 u evolution in time and by the 234 u 238 u o activity ratios that are generally within the range of sea water value of 1 14 0 04 2 sigma some dolomite rich samples are exceptional because their 234 u 238 u o ratios are significantly higher 1 22 1 38 than sea water suggesting deposition from anoxic pore waters where the soluble u reached anomalous 234 u 238 u ratios the formation of the carbonates from sea water derived fluids rather than from formation fluids advecting from deep aquifers is supported by the 87 sr 86 sr composition of the samples mean 0 709145 19 10 6 n 14 that compares well with modern nonseep marine carbonates and the ambient gulf of mexico sea water 0 709171 8 10 6 calcareous shells the 13 c values of which indicate a carbon source in sea water 13 c 4 3 to 1 1 yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates isotope migration attested by the observed u uptake in the fossil shells is likely to affect the accuracy of their ionium dates radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late pleistocene time 195 13 ka ages derived from nascent seep sites at mid bathyal and abyssal depths 12 3 0 0 ka indicate that currently vigorous seepage was initiated at the end of the last deglaciation these radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea floor

miocene reversal of bottom water flow along the pacific margin of the antarctic peninsula stratigraphic evidence from a contourite sedimentary tail a fossil mounded sedimentary body mb has been identified in the sedimentary record on the central continental rise west of adelaide island on the antarctic peninsula pacific margin the growth patterns of the mb are defined through a detailed regional stratigraphic analysis using multichannel seismic reflection profiles the mb has an elongated ne trend it overlaps and continues to the ne of an extensive cluster of seamounts and it developed between two non depositional troughs nine seismic units have been identified unit 9 the pre mb stage unit 8 mb growth stage units 7 and 6 mb maintenance stage units 5 and 4 transitional stage and units 3 2 and 1 inactive stage we interpret the mb as a patch drift plastered against the ne lee side of an obstacle as a long contourite sedimentary tail cst within a deep current that flowed northeastward this segment of the rise is however affected at present by a sw flowing branch of the lower circumpolar deep water lcdw from the weddell sea the depositional patterns of the mb growth and maintenance stages which are attributed an early miocene age on the basis of regional correlation of mcs profiles with dsdp site 325 and odp leg 178 drill sites provide the first evidence that bottom currents on the central continental rise flowed towards the ne at that time probably as part of the lower circumpolar deep water lcdw of the antarctic circumpolar current acc we suggest that significant palaeocirculation and palaeoceanographic changes occurred in this area and probably more widely during the middle miocene or at the miocene pliocene boundary although these results do not modify the regional stratigraphy of the major sediment drifts found on the continental rise of the antarctic peninsula s pacific margin they do indicate that the bottom current regime controlling the development of contourite deposits may have changed over time and also that more than one water mass has probably affected their distribution

was the laramide orogeny related to subduction of an oceanic plateau numerous models have been presented to explain the late cretaceous early cenozoic laramide orogeny which affected the foreland region of the western cordillera within the us1 the most attractive models invoke low angle subduction2 4 which can develop for various reasons5 6 evidence from south america indicates that one such reason may be the subduction of buoyant ocean floor we here extend the low angle subduction models by attributing the shallowing of the subduction angle during the laramide orogeny to subduction of a large oceanic plateau of anomalously thick and buoyant oceanic crust pacific plate history indicates that a twin to the hess rise may represent the oceanic plateau that triggered laramide events

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

classification of sedimentary furrows and a model for furrow initiation and evolution sedimentary furrows are longitudinal bed forms which form in fine grained cohesive sediments of the deep sea floor as well as in similar sediments in estuaries and in large lakes deep sea furrows have been observed where bottom currents often flow in one direction at 5 to 20 cm sec currents are much stronger greater than 50 cm sec in furrowed estuarine environments but sediment accumulation rates are higher a review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes those developed in muddy cohesive sediments tend to have steep walls and flat floors while those developed in carbonate rich cohesive sediments are broad and have gently sloping walls at abyssal depths furrows can often be detected by their distinctive echo patterns on 3 5 khz echo sounders a series of similar evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor however different furrow cross sections may generate distinctive hyperbolic echo patterns a model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring directionally stable and episodically strong currents furrows are initiated when secondary circulations in the boundary layer align coarse generally light debris in sand ribbon like bed forms as the coarse light sediments move along the sea floor they abrade small longitudinal troughs into the muds the grooves may fill in during intervals between strong current events if sedimentation rates are too high once a furrow is established on the sea floor further development may be controlled by the balance between sediment deposition and erosion where deposition occurs continuously and erosion occurs only episodically during brief periods of increased current strength narrow furrows width 1 5 to 1 15 of spacing appear to develop where deposition exceeds erosion whereas wide furrows width 1 2 of spacing develop where erosion exceeds or equals deposition dissolution may play an important role in the formation and evolution of furrows in carbonate rich cohesive sediments secondary circulation patterns over established furrows may be strongly controlled by bed form topography

evidence of abyssal eddies in the brazil basin 1 we report evidence of two deep cyclonic and two deep anticyclonic submesoscale eddies from world oceanographic circulation experiment hydrographic casts made in the brazil basin we infer that three of these were likely formed in or near the deep western boundary current dwbc of north atlantic deep water nadw and thus had traveled eastward after formation these eddies appear to be a new way for transporting nadw away from the dwbc to the ocean interior one of the apparent cyclonic eddies appeared to be laterally in contact with one of the anticyclonic eddies about 10 days later an attempt was made to resample the apparent eddies that had been in contact these observations although limited are interpreted to indicate that they survived the encounter that the cyclonic eddy had now moved to be beneath the anticyclonic one with each being somewhat thinner and that they produced a new anticyclonic eddy by partially merging deep float observations hogg and owens 1999 partially support the second inference

colorado plateau magmatism and uplift by warming of heterogeneous lithosphere the forces that drove rock uplift of the low relief high elevation tectonically stable colorado plateau are the subject of long standing debate while the adjacent basin and range province and rio grande rift province underwent cenozoic shortening followed by extension the plateau experienced approximately 2 km of rock uplift without significant internal deformation here we propose that warming of the thicker more iron depleted colorado plateau lithosphere over 35 40 myr following mid cenozoic removal of the farallon plate from beneath north america is the primary mechanism driving rock uplift in our model conductive re equilibration not only explains the rock uplift of the plateau but also provides a robust geodynamic interpretation of observed contrasts between the colorado plateau margins and the plateau interior in particular the model matches the encroachment of cenozoic magmatism from the margins towards the plateau interior at rates of 3 6 km myr 1 and is consistent with lower seismic velocities and more negative bouguer gravity at the margins than in the plateau interior we suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the colorado plateau and may be of general importance in plate interior settings

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

antarctic intermediate water circulation in the tropical and subtropical south atlantic recent hydrographic data from the south atlantic ventilation experiment cruises and others are combined with historical data and used to map the isopycnal properties corresponding to the antarctic intermediate water aaiw in the atlantic ocean the low salinity of the aaiw extends eastward across the south atlantic just south of the equator 3 4 s evidence of a weak eastward flow just north of the equator 1 2 n is also shown lateral and vertical homogenization of properties in the aaiw is found at the equator between 2 s and 2 n there is no clear zonal gradient in salinity just along the equator these observations suggest enhanced mixing within the equatorial baroclinic deformation radius the south atlantic tropical gyre is shown to consist of the following three cells one cyclonic cell centered at about 7 s another centered at about 19 s in the west and 23 s in the east and one anticyclonic cell centered at about 13 s these cells are associated with a westward extension at 10 s of high salinity and low oxygen which originates in the eastern tropical south atlantic and a front in these properties at about 15 s in the west and about 20 s in the east

regional three dimensional seismic velocity model of the crust and uppermost mantle of northern california 1 we present a three dimensional 3d tomographic model of the p wave velocity vp structure of northern california we employed a regional scale double difference tomography algorithm that incorporates a finite difference travel time calculator and spatial smoothing constraints arrival times from earthquakes and travel times from controlled source explosions recorded at network and or temporary stations were inverted for vp on a 3d grid with horizontal node spacing of 10 to 20 km and vertical node spacing of 3 to 8 km our model provides an unprecedented comprehensive view of the regional scale structure of northern california putting many previously identified features into a broader regional context and improving the resolution of a number of them and revealing a number of new features especially in the middle and lower crust that have never before been reported examples of the former include the complex subducting gorda slab a steep deeply penetrating fault beneath the sacramento river delta crustal low velocity zones beneath geysers clear lake and long valley and the high velocity ophiolite body underlying the great valley examples of the latter include mid crustal low velocity zones beneath mount shasta and north of lake tahoe

south atlantic fits and intraplate boundaries in africa and south america abstract the misfit problems raised by the pre drift reconstructions of the south and equatorial atlantic compel us to resort to intraplate deformations it is shown that acceptable deformations of the african plate in line with the benue trough and the transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the south atlantic these problems can instead be alleviated by considering a degree of deformation of the south american plate along an intraplate boundary in line with the parana province the rio grande rise and the walvis ridge in spite of the scarcity of field observations in this area of south america we show that such a deformation cannot be ruled out but it will have to be associated with deformations in the equatorial atlantic and in other areas of the african and south american plates

contourite depositional system on the argentine slope an exceptional record of the influence of antarctic water masses a significant contourite depositional system cds on the continental slope of the southern argentine margin is described here for the first time this system contains both erosive and depositional features that have resulted from several factors including topographic intensification of the antarctic sourced water masses the systematic northward decrease in speed of these water masses a northward increase of downslope sedimentary processes and local tectonic influences this system is an exceptional example of a cds that started to develop at the time of the eocene oligocene boundary potentially coeval with the opening of the drake passage however a new margin morphology characterized by a complex terraced slope lacking any continental rise developed after a major paleoceanographic change in the middle to late miocene we infer that this change resulted from the extension of north atlantic deep water circulation into the southern hemisphere and the deepening of antarctic bottom water circulation in the argentine basin

slope instabilities from echo character mapping along the french guiana transform margin and demerara abyssal plain the french guiana transform margin and demerara abyssal plain have been recently surveyed in the framework of the extraplac french program of extension of the continental shelf guyaplac survey ifremer ifp shom ipev based on the interpretation of some of the data collected during the guyaplac survey simrad em12 multibeam bathymetric data backscatter imagery and 3 5 khz profiles the area can be divided into three morphostructural domains 1 the western guiana margin including a part of the demerara plateau an important bathymetric relief prolonging the continental platform off guiana and surinam this domain is bounded by 1a the nw se trending northern border of the demerara plateau which appears quite steep and corresponds to a transform segment of the margin 1b the n s eastern border of the demerara plateau which corresponds to a divergent segment of the margin the demerara plateau shows a segmented morphology lowslope gradients and a very rough surface ripples perpendicular to the slope direction nnw sse structural steps seem to correspond to collapses of 100 km long blocs towards the east slumps initiate along these directions the observed rough bathymetry seems to be related to creeping processes at a greater scale seismic data this part of the margin has been totally destabilized numerous imbricate transparent masses rooted at about 0 5 s t w t t belowseafloor thenw se trending northern border of the demerara plateau corresponds to a cliff like continental slope probably slightly smoother than other transform margins ghana co te d ivoire margin the n s eastern border of demerara plateau is characterized by numerous small scale imbricate slumps some of these failures seem to be emplaced in the prolongation of the nnw sse structural steps identified on the demerara plateau 2 the eastern guiana margin corresponds to a nw se oriented gullied transform margin segment the associated continental slope is very steep and characterized by numerous imbricate slumps and related debris lows some undulated masses probably corresponding to creeping sediments or to older mass wasting events are still imprinted on bathymetry this transform margin segment is nearly entirely destabilized and eroded 3 the demerara abyssal plain this domain is characterized eastwards by channels belonging to the amazon turbidite system and westwards at the foot of demerara continental slope by sediment waves probably created by contour currents to conclude it seems that there is a strong relationship between the structure transform and divergent segments and the emplacement of recurrent slope instabilities these are probably related to the steepness of the slopes but also to subsidence histories generating in some cases huge deep seated collapses of the whole margin fluid ascents are common everywhere in the area probably enhancing slope instability their origin is not constrained but the black shales or cretaceous organic rich layers could be good candidates

multi process generated sediment waves on the landes plateau bay of biscay north atlantic detailed analyses of recently collected bathymetric and sparker seismic data support a new interpretation of the landes plateau field of sediment waves located on the aquitaine upper continental slope bay of biscay the wave geometry previously described as the result of a major sediment failure is interpreted as a structure with a complex origin including the interaction of depositional and gravity deformation processes depositional processes are mainly recorded by the upslope migrating pattern of the waves resulting from oblique or sigmoid downlap reflections on the upslope flank of the waves and by toplapping and truncated reflections on the downslope flank hemipelagic and turbiditic sedimentation may be involved in the wave building as well as contouritic processes that could be related to the existing northward polar current and internal waves gravity deformations are syndepositional discontinuous and of low amplitude affecting thick layers which alternate with undeformed layers they seem to correspond to gentle sediment creeping or stretching associated with minor listric or compaction like faults and possible limited back rotation of sediment blocks these multi process generated sediment waves could be rather common on the continental margins as they could have been mistaken with either depositional or deformational structures

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

the pleistocene to recent geological development of the ormen lange area a large volume of geological geophysical and geotechnical data have been acquired over the last decade in order to perform a safe development of the ormen lange gas field which is located in the scar of the holocene storegga slide the last in a series of large slides in this region over the last 500 ky based on high resolution seismic data and a series of geotechnical borings a unified pleistocene stratigraphy has been established for the area two main modes of deposition prevailed in the pleistocene during periods of peak glaciation when glaciers extended to the shelf break basal tills were deposited on the shelf and glacial debris flows on the continental slope during the much longer periods of reduced ice cover including interglacial periods normal marine and distal glacial marine deposition prevailed partly developed as contourites on the slope the resulting two sediment types are unsorted glacial diamictons and fine grained partly laminated sediments respectively these display distinctly different sedimentological and geotechnical properties and the failure planes for all the major slides are found in the fine grained marine deposits

origin of an enigmatic regional mio pliocene unconformity on the demerara plateau the demerara plateau located offshore french guiana and suriname is part of a passive transform continental margin particularly prone to develop slope instabilities probably in relation to the presence of a free distal border along its steep continental slope slope failure occurred at different periods cretaceous to neogene and shows an overall retrogressive evolution through time upslope these failure headscarp an enigmatic regional mio pliocene unconformity has been discovered through the interpretation of new academic and industrial datasets the aim of this work is to describe and understand the origin of this surface our analysis shows that this unconformity is made of a series of valleys that cross cut sedimentary strata each one of these valleys has a short lateral extent and is closed along two perpendicular directions which suggests that it could correspond to a highly meandering system or to some sub circular depressions the infill of these features is equivalent to the regional stratigraphic strata found outside the structures but in a subdued position this seems to imply that the structures have originated by a local loss of sediments at their base or by sliding processes furthermore these depressions intersect each other through time while migrating progressively downslope we discuss a series of hypotheses that try to explain the onset and evolution of these depressions forming the mio pliocene unconformity canyons slope failures contourite moats hydrate pockmarks having established that these structures are depressions formed by collapse and have many similarities with structures recently described in the literature as pockmarks associated with gas hydrate dissolution we favor this hypothesis we propose that these hydrate pockmarks form with a mass failure that was triggered by fluid overpressure development at the base of the hydrate stability zone

current controlled sedimentation in the equatorial atlantic examples from the southern margin of the guinea plateau and the romanche fracture zone abstract 3 5 khz and seismic reflection data were used in a study area of bottom current control of sedimentation in two areas of the equatorial atlantic a 100 km long segment of the romanche fracture zone rfz and a 30 km wide sector of the southern margin of the guinea plateau the rfz is the most important conduit for antarctic bottom water aabw into the eastern atlantic at mid depths ca 1500 4500 m north atlantic deep water nadw spreads southwards and eastwards whereas at the surface directly overlying the rfz is the equatorial divergence a zone of high biological production on the guinea margin a mixture of aabw nadw moving north washes the foot of the continental rise below 4250 m and the overlying nadw hugs the slope contours another divergence zone lies just to the north of this area on the southern margin of the guinea plateau sediment accumulation is controlled by contour currents within the nadw deposits are concentrated into a series of slope parallel sediment drift the surfaces of which are moulded into standing sediment waves in the slack region between areas of eroded or condensed sequences marking the loci of higher velocity current cores the outer edge and upper slope of the plateau under the influence of high velocities in the equatorial surface water are swept bare of unconsolidated sediment the lower rise sediments washed by the weak mixed aabw nadw water mass are generally flat surfaced in the rfz restricted sediment deposition and accumulation as well as the reworking of sediment surfaces document the influence of bottom currents flat surfaced sediments deposited on the floor of the axial valley below the ca 4500 m ccd are ponded into basins at the foot of the valley walls by a weak aabw on the northern flank of the north ridge pillsbury seamount and on the south ridge the overlying nadw has moulded the thin sediment cover into sediment waves a thicker accumulation of generally flat surfaced ponded sediments in the suspended valley were apparently deposited under a weaker current regime in the lee of the pillsbury seamount stronger bottom currents as well as steepness of slope appear to prevent sediment deposition on both the upper slope and crest of the north ridge

contourite drift types and their distribution in the north and south atlantic ocean basins abstract patterns of sediment accumulation under the control of deep bottom currents depend on interacting factors intensity of the regional deep circulation morphology of the sea floor coriolis effects abundance of terrigenous and biogenic supplies subsequent turbidity of deep nepheloid layers and depth of the ccd based on many examples of bottom current deposits from the atlantic and from other oceans we recognise three major types of contourite accumulation 1 giant elongate drifts 2 contourite sheets and 3 channel related drifts they generally occur in distinct topographic settings and show different geometries facies and rates of deposition in the north atlantic strong bottom circulation an open basin system and high sediment supply have led to the construction of large elongate contourite drifts which have striking relief 200 1000 m above the surrounding sea floor and are composed of muddy contourites rich in calcareous pelagic material in the south atlantic strong bottom currents are often trapped in gyre like circulation patterns around topographically closed basins or are accelerated through deep narrow connecting passages the resulting deposits are mainly extensive contourite sheets covered by fields of large sediment waves and channel related drifts including contourite fans constructed at the downstream exit of deep passages

structure and evolution of the demerara plateau offshore french guiana rifting tectonic inversion and post rift tilting at transform divergent margins intersection we present the structure and evolution of the eastern part of the demerara plateau offshore french guiana from the analysis of geophysical data collected during guyaplac cruise this area is located at the intersection of a transform segment and a divergent segment of a continental margin related to the early cretaceous opening of the equatorial atlantic the main structures are nne ssw to nnw sse trending normal faults on the eastern edge of the plateau and wnw ese to nw se trending acoustic basement ridges on its northern edge when replaced in their albian position these structures appear to be parallel to the coeval oceanic accretion axis and transform faults respectively the most striking structures are related to a post rift but syn transform tectonic inversion producing e w to wnw ese trending folds sealed by a regional unconformity this shortening can not be related to ridge push but is probably related to a plate kinematic change 105 my ago that modified the deformation in the vicinity of the transform fault late post rift evolution also includes a significant tertiary oceanward tilt of the edge of the demerara plateau the driving mechanism of this late tilt is unclear but may be related to a lithospheric flexure resulting from the loading of the abyssal plain by the orinoco and amazon deep sea fans

abyssal circulation change in the equatorial atlantic evidence from cenozoic sedimentary drifts off west africa abstract seismic profiling in the equatorial atlantic reveals deep water 4500 m sediment bodies formed by current controlled deposition near the intersection of large offset fracture zones with the african margin a 600 km long drift accumulation the ivory coast rise lies north of the st paul transform near 3 n a smaller drift deposit has been identified along the northern side of the guinea transform at 10 n antarctic bottom water aabw which presently enters the eastern atlantic basins through the romanche and vema fracture zones probably played an important role in the development of these features proto aabw may have reached the equatorial region as early as mid eocene time before the establishment of permanent ice sheets in antarctica the ivory coast rise existed as a distinct sedimentary drift by the mid eocene as a result of deposition from bottom water moving southwards along the african margin and westwards parallel to the st paul fracture zone this early flow pattern is in the opposite sense to the present movement of deep water in the sierra leone basin a reversal in abyssal circulation may have been caused by the northward passage of the region across the paleoequator during the cenozoic

impact of rock uplift on rates of late cenozoic rocky mountain river incision 1 the high relief of the modern rocky mountain landscape formed in the late cenozoic by downcutting of a fluvial network that links a series of easily eroded sedimentary basins across relatively resistant crystalline cores of adjacent ranges using a numerical model of fluvial erosion and the flexural isostatic response to the associated unloading we first calculate the expected pattern and pace of incision caused by rock uplift related to migration of the yellowstone hot spot and to growth of the northern portion of the rio grande rift calculated incision rates are 60 m myr and total depth of erosion of sedimentary basins is 300 m well below the long term incision rates and amounts of erosion interpreted from the geologic record broad scale tilting of the region toward the east accomplished by a gradient in rock uplift of 1 km along the north south axis of the central rockies declining to zero 1000 km to the east can account for the additional erosion needed to match observations in each modeling scenario stream incision is nonsteady with rock uplift outpacing erosion for 1 myr in perimeter basins and 1 5 myr in interior basins three factors dominate the spatial and temporal pattern of regional landscape evolution 1 the time since uplift began 2 the uplift pattern and 3 the distribution of relatively resistant bedrock within the region our results suggest that the spatial variability in late cenozoic exhumation can be explained by a long lived transience in the stream network response to these various late cenozoic geophysical events

crustal genesis on the oregon plateau the areal and temporal distribution and the chemical and isotopic characteristics of cenozoic volcanic rocks from the oregon plateau reflect the changing tectonic conditions affecting this area over the last 20 m y following subduction zone related calc alkaline volcanism flood basalt eruptions occurred in the time interval 18 14 ma this primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the juan de fuca ridge that resulted in changes in the geometry of subduction against the pacific northwest the compositions of the mid miocene flood basalts show signs of extensive crystal fractionation in some cases accompanied by crustal contamination roughly 11 m y ago basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions this implies that as extension in the oregon plateau proceeded the crust became less of an impediment to the passage of relatively unfractionated mantle melts initial sr nd and pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area these observations suggest that an area of some 130 000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern north america over the last 20 m y the intensity of basalt volcanism as manifest in the columbia river group and in the oregon plateau does not appear to reflect the presence of a hot spot beneath the area both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by back arc spreading in a manner analogous to the formation of the marginal basins of the western pacific this spreading initiated along the western border of the archean wyoming craton with the location of mantle ascent melt production and extraction controlled by the changing geometry of subduction the existence of a zone of lithospheric weakness and the presence of a thick low density subcontinental mantle keel existing beneath the archean craton in this area

significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data how and where the ocean tides dissipate their energy are long standing questions1 that have consequences ranging from the history of the moon2 to the mixing of the oceans3 historically the principal sink of tidal energy has been thought to be bottom friction in shallow seas4 5 there has long been suggestive evidence6 7 however that tidal dissipation also occurs in the open ocean through the scattering by ocean bottom topography of surface tides into internal waves but estimates of the magnitude of this possible sink have varied widely3 8 9 10 11 here we use satellite altimeter data from topex poseidon to map empirically the tidal energy dissipation we show that approximately 1012 watts that is 1 tw representing 25 30 of the total dissipation occurs in the deep ocean generally near areas of rough topography of the estimated 2 tw of mixing energy required to maintain the large scale thermohaline circulation of the ocean12 one half could therefore be provided by the tides with the other half coming from action13 on the surface of the ocean

cortex a shipboard xrf scanner for element analyses in split sediment cores abstract the cortex corescanner texel is a non destructive xrf system for the logging of split sediment cores it is a seaworthy instrument that remains active during heavy weather conditions the scanner produces reliable counts for k to sr highly correlated with concentrations measured with atomic absorption spectrometry aas the option to scan al si and elements heavier than sr is in preparation a core section of 1 m can be scanned at 2 cm intervals within an hour a maximum resolution of 1 mm can be achieved at present each region and sedimentary environment requires separate calibration in the nearby future the generator settings will be standardized which may result in the development of a standard calibration procedure because of the high resolution of its semi quantitative records the cortex is a useful tool for detailed time series analysis inter core correlation and shipboard analysis

distinguishing sediment waves from slope failure deposits field examples including the humboldt slide and modelling results abstract migrating sediment waves have been reported in a variety of marine settings including submarine levee fan systems floors of fjords and other basin or continental slope environments examination of such wave fields reveals nine diagnostic characteristics when these characteristics are applied to several features previously attributed to submarine landslide deformation they suggest that the features should most likely be reinterpreted as migrating sediment wave fields sites that have been reinterpreted include the humboldt slide on the eel river margin in northern california the continental slope in the gulf of cadiz the continental shelf off the malaspina glacier in the gulf of alaska and the adriatic shelf a reassessment of all four features strongly suggests that numerous turbidity currents separated by intervals of ambient hemipelagic sedimentation deposited the wave fields over thousands of years a numerical model of hyperpycnal discharge from the eel river for example shows that under certain alongshore current conditions such events can produce turbidity currents that flow across the humboldt slide serving as the mechanism for the development of migrating sediment waves numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor i e numerical steps sediment waves can form and migrate upslope hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms the morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field including those that were previously described as submarine landslides

the guinea continental margin an example of a structurally complex transform margin abstract the guinean continental margin corresponds to a wide hinge zone which developed between the jurassic central atlantic and the cretaceous equatorial atlantic the guinea marginal plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the early cretaceous on the basis of seismic profiles recorded in two selected areas located along the southern guinea margin a seismic stratigraphy is established at depth the plateau is underlain by a thick mesozoic and cenozoic sedimentary wedge made of two main ensembles a lower ensemble or sequence 1 consists of early cretaceous clastics an upper ensemble includes a series of four distinct sequences attributed to albian to tertiary deposits eastward the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of paleocene age beneath the southern margin of the guinea plateau the sedimentary sequences appear deformed by faults and folds resulting from at least two main tectonic episodes a first event only affected sequence 1 which is cut by a set of extensional faults a second tectonic phase including folding reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2 during this tectonic episode former normal faults have been reactivated as reverse faults resulting in a structural inversion a third tectonic phase was responsible for structures transverse to the general e w trend of the southern guinea margin and consisting of n s trending normal faults and associated volcanoes the spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation a first stage is characterized by a divergent rifting leading to n s trending synsedimentary normal faults during the cretaceous the tectonic activity increased as a general transform motion occurred between the guinea plateau and its american conjugate margin the demerara plateau local tensional stresses were responsible for a splay fault system active near the tip of the guinea plateau in late albian times a slight adjustment of the africa plate with respect to the south american one resulted in local squeezing of the whole sedimentary wedge after this short compressive event final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity

seismic heterogeneity and small scale convection in the southern california upper mantle we invert traveltime residuals of teleseismic p and s phases for 3 d perturbations in vp vs and vp vs structures in the southern california upper mantle the tomographic inversion uses frequency dependent 3 d sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals in addition to separate vp and vs tomography we jointly invert the p and s data sets for vp vs perturbations by imposing a smoothness constraint on the lnvs lnvp field the regional upper mantle is very heterogeneous with the greatest amplitude of velocity and vp vs perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern california our imaging improves the resolution of the geometry and amplitude of these features in addition to the major structures imaged and discussed in previous papers we find a large high velocity anomaly at depths between 340 and 500 km beneath the borderland transverse ranges and northern peninsular ranges this anomaly is separate from the well known uppermost mantle transverse ranges anomaly which extends no deeper than 175 200 km the strongest low velocity high vp vs anomaly is found beneath and west of the salton trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning the magnitudes of vp vs and vp vs perturbations and knowledge of regional 1 d velocity and attenuation lead us to conclude that the asthenosphere contains up to 1 partial melt extending to depths as great as 150 200 km

elongate depressions on the southern slope of the norwegian trench skagerrak morphology and evolution abstract multibeam echosounder surveys conducted on the southern slope of the norwegian trench between norway and denmark show that elongate depressions interpreted as current modified pockmarks are widespread in water depths of 100 500 m in the holocene recent succession the long axes of the depressions are oriented at an angle of about 15 downslope relative to the depth contours the long axes appear to almost exactly parallel the effective bottom currents in this part of the skagerrak the present current directions were probably established about 4000 years ago following a major shift in the oceanographic circulation pattern the most characteristic feature of the depressions is their large size with maximum depths of 45 m widths of 400 m and lengths of 2 km strong bottom currents continuously transport particles thrown into suspension by the seepage process away from the pockmarks and prevent further deposition continuous seepage of shallow gas at fixed pockmark sites over long time periods combined with generally high sedimentation rates are thought to be responsible for the exceptional size of many depressions the shallow gas probably has a mixed biogenic thermogenic origin however the thermogenic component is considered to be most important for the formation of pockmarks seepage from permeable layers in the jurassic cretaceous bedrock is evidenced by linear rows of depressions above these horizons while pockmarks above sub surface faults indicate gas seeps also from there in shallow water areas the shape and size of pockmarks are locally dependent on their location with respect to giant sand waves which trend sub parallel to the depth contours and are interpreted to be of late weichselian to early holocene age in deeper water areas the pockmarks have been further modified by gravity slides caused by collapse of steep pockmark walls

mass transport deposits and fluid venting in a transform margin setting the eastern demerara plateau french guiana abstract the eastern demerara plateau offshore french guiana was surveyed in 2003 during the guyaplac cruise multibeam bathymetry and acoustic imagery 6 channel seismic reflection and 3 5 khz echo sounding the data show the post transform cenozoic that the series located on the outer part of the plateau below c 2000 m contain at least twelve stacked mass transport deposits mtds that have recorded a history of large scale slope failure as well as two main normal fault sets that provide possible pathways for upward fluid migration through the series reaching at high as the uppermost mtds seabed data show that the area above the failures is characterized by circular to elongate slope parallel depressions interpreted as fluid seeps pockmarks some of them have been modified by along slope currents we suggest that the development of the mtds to results from the combinaiton of the presence of fluid overpressure at depth the geometry of the margin s deep structure in particular the existence of a free borderlateral border on the outermost plateau our results also emphasise the role of stratigraphic decollements within the cenozoic series

bedform velocity matrix the estimation of bottom current velocity from bedform observations a wide variety of bedforms both depositional and erosional in origin has been recognized on the deep seafloor and attributed to the influence of bottom currents these range in scale from those visible in bottom photographs centimeter to decimeter to those recorded with seafloor bathymetric imaging meter to kilometer in many cases it has been possible to provide some quantification of substrate grain size and flow velocity responsible for each bedform type we have synthesized both our own and published data in order to present a bedform velocity matrix which facilitates the estimation of bottom current velocity based on bedform type despite imperfections we believe this to be a valuable model for assessing strength and variability of bottom currents that can have a significant influence on the siting of submarine cables pipelines and other seafloor installations

the eocene late pliocene paleoenvironment in the voring plateau area norwegian sea paleoceanographic implications abstract from high resolution and multichannel seismic data integrated with the relevant odp and dsdp core sites the sediments on the voring plateau offshore mid norway was studied four seismic units were identified units vd eocene vc oligocene vb miocene middle pliocene and va late pliocene pleistocene the history of sedimentation was controlled by two main factors subsidence and deposition in deeper water as the study area moved away from the active mid oceanic spreading ridge and the pattern and development of the ocean circulation system during the eocene ocean current influenced sedimentation prevailed on the outer plateau slope while shallow marine conditions prevailed on the inner parts the oligocene was dominated by ocean current influenced sedimentation on the northeastern inner high which also affected the southern part from the early miocene coeval with renewed influence of ocean current controlled sedimentation on the outer slope sometime during the late miocene to early pliocene period a change to hemipelagic sedimentation occurred indicating that the topographic control of the voring marginal high on the ocean currents was reduced the paleogene ocean circulation system may have been interbasinal and restricted to the norwegian greenland sea or intrabasinal and connected to the north atlantic from our data it is not possible to discriminate between the two alternatives during the neogene the most important change was controlled by the subsidence of the voring marginal high causing northeastward flowing water masses to flow along the shelf and slope of mainland norway

the global attenuation structure of the upper mantle 1 a large data set of fundamental mode rayleigh wave amplitudes is analyzed to derive a new global three dimensional model of shear wave attenuation in the upper mantle the amplitude observations span a range of periods between 50 and 250 s and are derived from earthquakes with mw 6 0 that occurred between 1993 and 2005 four separate factors may influence an amplitude anomaly intrinsic attenuation along the raypath elastic focusing effects along the raypath uncertainties in the strength of excitation and uncertainties in the response at the station in an earlier paper dalton and ekstrom 2006a dependence of the retrieved attenuation structure on these terms was shown to be significant and an approach was developed to invert the amplitudes simultaneously for each term the new three dimensional attenuation model qrfsi12 which is the subject of this paper is derived using this method the model contains large lateral variations in upper mantle attenuation 60 to 100 and exhibits strong agreement with surface tectonic features at depths shallower than 200 km at greater depth qrfsi12 is dominated by high attenuation in the southeastern pacific and eastern africa and low attenuation along many subduction zones in the western pacific resolution tests confirm that the change in pattern of attenuation above and below 200 km depth can be determined with confidence using the fundamental mode data set the new model is highly correlated with global models of shear wave velocity particularly in the uppermost mantle suggesting that the same factors may control both seismic attenuation and velocity in this depth range however forcing the lateral perturbations in attenuation to match those found in global velocity models decreases the data variance reduction which suggests that subtle differences between patterns of attenuation and velocity are robust

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

contourites and their relevance for mass wasting along the mid norwegian margin between about 12 and 4 3 ma slope parallel bottom currents dominated the neogene sedimentation on the slope of the mid norwegian margin and large scale sediment bodies accumulated to thicknesses up to 1000 m these bedforms are morphologically classified as sheeted and elongated mounded contourite drifts an increase in sediment supply from about 4 to 3 ma due to onshore uplift and glaciation resulted in a massive influx of downslope derived material onto the continental margin only on the voring plateau was contourite sedimentation little affected by this change in sedimentary style during the last 0 5 my contouritic deposits occurred also in the storegga area as 100 150 m thick infill drifts inside the scars of two palaeo slides r and s holocene post slide deposits in the scar of the storegga slide 8 2 ka have the typical morphological features and distribution of contourite drifts with the upslope limit at the lower base of the warm northward flowing north atlantic current at 500 700 m depth and the downslope limit at depths of approximately 1200 m detailed chronostratigraphy of the post slide sediments show relatively constant high sedimentation rates in the order of 1 2 m ky during the last 8000 years this is equivalent to the estimated rates for the older infill drifts the infill drifts consist mainly of clays with silty and sandy lamina and represents marine and glaciomarine conditions the main influence of contourite drifts on slope stability relates to their physical properties forming a more sensitive and brittle deposit than the glacial deposits and their exposure to downslope sedimentation in addition the potential for build up of excess pore pressure due to rapid loading from subsequent glacial depositions their seabed smoothing effect and great lateral extent make the contourite drift deposits particularly important for slope instability in this region

p and s waveform modeling of continental sub lithospheric detachment at the eastern edge of the rio grande rift 1 east of the rio grande rift tomographic images of teleseismic data have revealed a se dipping slab like structure underneath the western edge of the great plains in the southwestern united states however finite difference synthetics require an amplified tomographic model to reproduce the waveform distortions as observed in broadband waveform data recorded along the la ristra transect in addition to travel time anomalies song and helmberger 2007 demonstrated how to use s waveforms and their amplitude patterns to further constrain the magnitude of the anomalous structure their preferred s velocity model suggests that the slab like structure is about 4 fast 120 km thick and dipping 70 75 to the se to about a depth of 600 km we adapt the preferred s wave model from song and helmberger 2007 and scale the p wave model using a suite of scaling factors sf dlnvs dlnvp we find that synthetics from the p model generated by sf 1 25 can distort p waveforms and fit the amplitude pattern best such a low sf indicates that the slab like anomaly is not only cold but also compositionally distinct we make use of sf and the s wave anomaly simultaneously to obtain corresponding vp vs anomaly and infer the origin of the slab like anomaly our result suggests that the observed sub lithospheric detachment is 310 20 c colder and more depleted than the adjoining mantle asthenosphere by 3 units of mg mg mg fe 100 it is negatively buoyant and is geodynamically consistent with the observed foundering of the continental lithosphere at the eastern edge of the rio grande rift in short we demonstrate the importance of seismic waveform anomalies in geochemical and geodynamic inferences

a synthesis of the sedimentary evolution of the demerara plateau central atlantic ocean from the late albian to the holocene abstract the demerara plateau off the french guiana and surinam margin corresponds to a prominent sub horizontal marginal plateau the interpretation of a large set of seismic data acquired along the demerara plateau and integration with previous seismic stratigraphy and drill studies enables a better understanding of the sedimentary evolution of the outer demerara marginal plateau since the end of the transform activity late albian three sedimentary evolutionary stages have been identified 1 the pre contourite stage late albian to early miocene is strongly controlled by the structure of the margin that influences the distribution pattern of post transform deposits through differential thermal subsidence between transform and divergent segments 2 the transitional stage middle miocene to early pliocene is expressed as a current controlled erosive surface resulting from the onset of bottom current interactions due to the establishment and strengthening of the modern thermohaline circulation that is coeval with the progressive closure of the isthmus of panama 3 the contourite stage middle pliocene to holocene is characterized by the development of a contourite depositional system that is linked to the activity of the north atlantic deep water circulation and by a complex interaction between alongslope and downslope processes our study highlights that marginal plateaus might be particularly prone to develop contourite features since they regionally influence ocean current dynamics in guiding and accelerating bottom contour currents marginal plateaus should thus be considered as a key target to study bottom current dynamics and paleocirculation patterns

continent wide drainage reorganization in north america driven by mantle flow abstract north america na experienced pronounced changes in continental scale drainage characterized by a reversal for much of the continental interior from north into the canadian arctic to south into the gulf of mexico gom from the mid cretaceous to the paleocene however the driving mechanism for these profound drainage reorganizations remain unexplained here we investigate the role of mantle flow on landscape evolution by coupling dynamic topography with surface processes this approach enables us to simulate catchment dynamics and the rearrangement of sediment transport in response to mantle flow we show that a west to east drainage reversal can be induced by the na overriding the subducted farallon plate moreover augmented dynamic subsidence caused by a basalt to eclogite transformation of an oceanic plateau within the farallon slab depressed the gom region and expanded the integrated drainage to the gom since the early paleocene for the first time we show that dynamic topography can explain the north to south continental scale drainage reorganization in north america