the importance of upper mantle heterogeneity in generating the indian ocean geoid low one of the most pronounced geoid lows on earth lies in the indian ocean just south of the indian peninsula several theories have been proposed to explain this geoid low most of which invoke past subduction some recent studies have also argued that high velocity anomalies in the lower mantle coupled with low velocity anomalies in the upper mantle are responsible for these negative geoid anomalies however there is no general consensus regarding the source of this particular anomaly we investigate the source of this geoid low by using models of density driven mantle convection our study is the first to successfully explain the occurrence of this anomaly using a global convection model driven by present day density anomalies derived from tomography we test various tomography models in our flow calculations with different radial and lateral viscosity variations some of them produce a fairly high correlation to the global geoid but only a few smean2 gypsum semucb and llnl jps could match the precise location and pattern of the geoid low in the indian ocean the source of this low stems from a low density anomaly stretching from a depth of 300 km down to similar to 900 km in the northern indian ocean region this density anomaly potentially originates from material rising along the edge of the african large low shear velocity province and moving toward the northeast facilitated by the movement of the indian plate in the same direction plain language summary one of the lowest geoid anomalies on earth lies in the indian ocean just south of the indian peninsula several theories have been proposed to explain this gravity low most of these theories try to explain the existence of this anomaly with the help of cold dense oceanic plate that sank into the mantle in the past and which could potentially be present below the indian ocean at depths greater than 1 000 km however there is no general consensus regarding the source of this particular negative geoid anomaly we investigate the source of this low by using models of density driven mantle convection our study finds that the source of this low stems from a low density anomaly stretching from a depth of 300 km down to similar to 900 km in the northern indian ocean region this density anomaly potentially originates from hot buoyant material rising from deep mantle beneath africa and moving toward the northeast facilitated by the movement of the indian plate in the same direction our study is the first to successfully explain the occurrence of this geoid low using present day density anomalies

a magnetotelluric study of the alpine fault new zealand summary magnetotelluric soundings have been made at seven locations on a 4 km profile crossing the alpine fault in the south island of new zealand the distortion techniques of groom bailey 1989 and lilley 1998a b have been used to derive regional apparent resistivity and phase curves that correspond to electromagnetic induction in orientations parallel and perpendicular to the fault 2 d inversion of the regional responses reveals that a narrow 1 km wide conductive zone is associated with the alpine fault this conductor is most probably related to the heating of deep circulating meteoric water in a region in which enhanced temperatures occur at shallow depth due to the tectonic uplift of the southern alps

downward particle fluxes in the guadiaro submarine canyon depositional system north western alboran sea a river flood dominated system abstract three moorings equipped with sediment traps were deployed in the north western alboran continental margin to study downward particle fluxes in the guadiaro submarine canyon depositional system this area is located close to the strait of gibraltar and is influenced by the upwelling induced by the atlantic jet and by episodic flood events from the guadiaro river sediment traps were installed in the guadiaro canyon in the guadiaro channel and in the adjacent continental slope the overall duration of the deployment was 12 months from november 1997 to october 1998 time series of downward particle fluxes major constituents organic carbon nitrogen biogenic opal calcium carbonate and lithogenics and 210pb were determined near the surface at mid depths and near the bottom total mass fluxes tmf in this area fluctuated more than two orders of magnitude and showed an important seasonal variability with higher fluxes in winter increases in tmf and lithogenics together with decreases in 210pb organic carbon and opal were recorded in all traps coinciding with river floods indicating a direct response of the system to these events and a rapid offshore transport of suspended matter affecting the entire water column the channel site received similar particle fluxes to the western open slope site indicating that this channel did not act as a preferential sediment conduit during the deployment period in the guadiaro canyon tmf were more than one order of magnitude higher 210pb concentration was lower one half and organic matter was more degraded than at the channel site during spring and summer as a consequence of receiving particles resuspended by internal waves and occasionally by trawling activities these particles were mainly retained in the canyon which works as a trap also during spring and summer the opal and organic carbon percentages increased in all traps both in magnitude and variability and peaks seem to be associated with biological blooms

the relationships between large scale variations in shear velocity density and compressional velocity in the earth s mantle a large data set of surface wave phase anomalies body wave travel times normal mode splitting functions and long period waveforms is used to investigate the scaling between shear velocity density and compressional velocity in the earth s mantle we introduce a methodology that allows construction of joint models with various levels of scaling complexity d ln d lnvs d lnvs d lnvp in order to detect seismological signatures of chemical heterogeneity we demonstrate that the data sets considered cannot be fit concurrently with a uniform or a positive and uniform throughout the mantle the variance reductions to p wave travel times and vp sensitive modes are up to 40 higher with our preferred model of anisotropic shear and compressional velocity than the recent anisotropic shear velocity model s362ani m which was constructed assuming a uniform throughout the mantle several features reported in earlier tomographic studies persist after the inclusion of new and larger data sets anticorrelation between bulk sound and shear velocities in the lowermost mantle as well as an increase in with depth in the lower mantle are largely independent of the regularization scheme when correlations between density and shear velocity variations are imposed in the lowermost mantle variance reductions of several spheroidal and toroidal modes deteriorate by as much as 40 recent measurements of the splitting of 0s2 in particular are largely incompatible with perfectly correlated shear velocity and density heterogeneity throughout the mantle a way to significantly improve the fits to various data sets is by allowing independent density perturbations in the lowermost mantle our preferred joint model consists of denser than average anomalies 1 peak to peak at the base of the mantle roughly coincident with the low velocity superplumes the relative variation of shear velocity density and compressional velocity in our study disfavors a purely thermal contribution to heterogeneity in the lowermost mantle with implications for the long term stability and evolution of superplumes

the adjustment of mantle plumes to changes in plate motion the relative motion of hotspots and lithospheric plates implies a velocity shear in the underlying mantle causing horizontal advection of mantle plumes as they rise toward the lithosphere consequent tilting of plumes parallel to the direction of plate motion indicates that plumes must undergo a period of readjustment after the velocity vector for plate motion is altered thus the shape of bends in the surface tracks of hotspots resulting from changes in plate motion will reflect the plume adjustment laboratory experiments as well as computations using a simple theory developed in richards griffiths 1988 for the dynamics of continuous plume conduits demonstrate that the bend in the surface track has a radius of curvature approximately equal to the maximum horizontal deflection of the conduit thus the sharpness of the bend at an age of 43ma in the hawaiian emperor volcanic chain implies that the deflection of the underlying plume in that case was small 200 km this small deflection is expected for plumes carrying large buoyancy fluxes and it indicates that tilting of the conduit is unlikely to be sufficient to cause diapiric instability

observations of the internal tide in monterey canyon abstract data from two shipboard experiments in 1994 designed to observe the semidiurnal internal tide in monterey canyon reveal semidiurnal currents of about 20 cm s 1 which is an order of magnitude larger than the estimated barotropic tidal currents the kinetic and potential energy evidenced by isopycnal displacements of about 50 m was greatest along paths following the characteristics calculated from linear theory these energy ray paths are oriented nearly parallel to the canyon floor and may originate from large bathymetric features beyond the mouth of monterey bay energy propagated shoreward during the april experiment itex1 whereas a standing wave that is an internal seiche was observed in october itex2 the difference is attributed to changes in stratification between the two experiments higher energy levels were present during itex1 which took place near the spring phase of the fortnightly 14 8 days cycle in sea level while itex2 occurred close to the neap phase further evide

geomorphic characterization of four shelf sourced submarine canyons along the u s mid atlantic continental margin abstract shelf sourced submarine canyons are common features of continental margins and are fundamental to deep sea sedimentary systems despite their geomorphic and geologic significance relatively few passive margin shelf breaching canyons worldwide have been mapped using modern geophysical methods between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the us mid atlantic margin wilmington baltimore washington and norfolk canyons more than 5700 km2 of high resolution multibeam bathymetry and 890 line km of sub bottom chirp profiles were collected along the outer shelf and uppermost slope depths of 80 1200 m the data allowed us to compare and contrast the fine scale morphology of each canyon system the canyons have marked differences in the morphology and orientation of canyon heads steepness and density of sidewall gullies and the character of the continental shelf surrounding canyon rims down canyon axial profiles for washington baltimore and wilmington canyons have linear shapes and each canyon thalweg exhibits morphological evidence for recent relatively small scale sediment transport for example washington canyon displays extremely steep wall gradients and contains 100 m wide 5 10 m deep v shaped incisions down the canyon axis suggesting modern or recent sediment transport in contrast the convex axial thalweg profile the absence of thalweg incision and evidence for sediment infilling at the canyon head suggest that depositional processes strongly influence norfolk canyon during the current sea level high stand the north walls of wilmington washington and norfolk canyons are steeper than the south walls due to differential erosion though the underlying cause for this asymmetry is not clear furthermore we speculate that most of the geomorphic features observed within the canyons e g terraces tributary canyons gullies and hanging valleys were formed during the pleistocene and show only subtle modification by holocene processes active during the present sea level high stand

submarine canyons and channels in the lower st lawrence estuary eastern canada morphology classification and recent sediment dynamics abstract series of submarine canyons and channels observed in the lower st lawrence estuary lsle provide an opportunity to analyze in great detail the morphology spatial distribution and modern activity of such systems in a relatively shallow 300 m semi enclosed basin based on their geomorphology and physical settings the canyons and channels were classified into four categories according to their feeding sources ancient or recent glacially fed river fed longshore drift fed and sediment starved systems their activity was interpreted based on geomorphological characteristics such as the presence of bedforms related to gravity flows backscatter intensity axial incision and the presence of rapidly deposited layers in surficial sediments river fed deltas were interpreted as inactive mainly because suspended sediment concentrations at river mouths are low preventing the generation of hyperpycnal currents or delta lip failures related to high sediment supply longshore drift fed canyons present where the coastal shelf narrows were found to be episodically active probably due to earthquakes or extreme storm events unlike other longshore drift fed canyons observed elsewhere in the world they are active infrequently because of the modern low rates of sediment supply to their heads the most active canyons are the sediment starved type and were observed near pointe des monts their activity is probably due to slope failures and to the presence of strong hydrodynamic processes therefore sediment supply is not the main mechanism responsible for modern canyon and channel activity in the lsle however sediment supply has been an important factor during the formation of the submarine channels and canyons their quasi exclusive occurrence on the quebec north shore is attributed to its larger watershed and important sedimentary delivery during deglaciation the quebec north shore watershed is 20 times greater than the quebec south shore watershed which favored the transport of greater volumes of sediment during the early holocene moreover the slope proximity to the shore led to the formation of longshore drift fed systems on the north shore when sediment supplied to rivers were transferred on a narrow shelf

characteristic patterns of the circulation in the santa barbara channel the subtidal near surface circulation in the santa barbara channel sbc and on the shelf north of point conception is described based on observations obtained during the 3 year period from 1993 to 1995 near surface currents in the channel are a superposition of a larger than sbc scale flow and a cyclonic circulation of variable intensity located inside the channel on seasonal timescales the larger than sbc scale flow near the surface is equatorward in spring and poleward from summer through winter the increase in equatorward flow in spring occurs concurrently with the increase in equatorward wind stress and the decrease in near surface temperatures and synthetic subsurface pressures ssps the flow reverses in late spring simultaneously with the increase in the along channel ssp difference and months before wind stress has reached its peak the period during which the cyclonic circulation within the sbc is strongest coincides with the period of strongest poleward flow through the eastern entrance a synoptic description of the circulation in the sbc is presented in terms of six characteristic patterns labeled upwelling relaxation cyclonic propagating cyclones flood east and flood west an analysis of the 5 and 45 m currents into empirical orthogonal functions eofs isolates 50 53 of the 5 m 45 m low frequency current variance into three two modes combining these modes with the mean current fields the modes have spatial patterns that correspond to the characteristic flow patterns subjectively deduced from inspection of daily averages of the near surface currents from late spring through fall the two largest 5 m current modes produce a repeating pattern of circulation in which the sequence of four states namely upwelling cyclonic relaxation and quiescent is traversed roughly every 16 days in addition to the large scale cyclonicity in the central sbc smaller cyclonic eddies form frequently in the eastern channel and travel toward the west with an average speed of 0 06 m s 1 the patterns described here develop in response to the wind stress and the along channel ssp gradient

lateral variations in mantle rheology implications for convection related surface observables and inferred viscosity models summary over the past decade numerous analyses of convection related observables such as horizontal surface divergence geoid or gravity anomalies and dynamic surface topography have been carried out in the context of tomography based mantle flow models one of the major objectives of this modelling has been the inference of the rheological structure of the mantle with few exceptions these studies have been conducted in the framework of a viscous flow theory which assumes that the mantle rheology may be represented in terms of an effective viscosity which varies with depth only here we present a detailed assessment of the impact of lateral variations in viscosity on global convection related observables using forward modelling of buoyancy induced flow in a 3 d spherical shell we find that the resulting dynamic topography at the surface and the core mantle boundary as well as the gravitational response of the earth are affected relatively little by the inclusion of lateral viscosity variations lvv when compared with results for a purely 1 d radial viscosity model in particular we found that the effect of lvv on the global observables is significantly smaller than the variability due to uncertainties in the current seismic tomography models we also quantify the effect of lvv in the context of the viscosity inverse problem using two synthetic data sets generated with a 1 d viscosity profile and with a fully 3 d viscosity model in which the lvv span across three orders of magnitude we compared the 1 d viscosity profiles recovered from the inversions and found that lvv have virtually no effect on our inversion results the synthetic viscosity inversion further revealed that the effect of lvv is small in comparison to the uncertainties arising from the seismic tomography models the inversions also suggest that the 1 d viscosity profiles derived from actual surface data represent the depth variation of the horizontally averaged logarithm of the 3 d viscosity distribution in the mantle intr o duction constraining the rheological properties of the mantle using surface geophysical data has traditionally involved separate treatments of short seismic intermediate solid earth tides long postglacial rebound and very long mantle convection timescale processes in each case the initial focus historically speaking has been on the inversion of the data to determine a one dimensional 1 d purely radial variation of the relevant rheological parameter e g elastic moduli intrinsic attenuation and viscosity which provides the best fit to the data it has long been recognized particularly in the context of the seismic inverse problem that the search for a 1 d earth model is subject to possible bias due to the presence of lateral variations in structure which may be as significant as the radial variations especially at shallow depths this paper is concerned with the mantle viscosity field the earliest inferences of the radial i e horizontally averaged profile of viscosity based on glacial isostatic adjustment gia data e g mcconnell 1968 parsons 1972 were characterized by a pronounced increase of viscosity with depth in the top half of the mantle however these were followed by two decades of gia research that argued for an essentially isoviscous

limited true polar wander as evidence that earth s nonhydrostatic shape is persistently triaxial earth s spin axis follows the maximum moment of inertia axis of mantle convection with some delay due to adjustment of the rotational bulge here we compute this axis for geodynamic models based on subduction history assuming constant slab sinking speed with another contribution due to thermochemical piles for a wide range of parameters a large shift of 90 degrees is predicted around 80 90 ma it can be largely attributed to a change in circum pacific subduction from predominantly in the north and south towards east and west actual amounts of true polar wander are much smaller pointing towards additional inertia tensor contributions possibly due to slabs in the lowermost mantle below both polar regions these slabs would have been subducted before 150 ma when plate motions in the panthalassa basin are largely unknown matching predicted and observed true polar wander can serve at constraining such plate motions

climate change and the episodicity of sediment flux of small california rivers abstract we studied the streamflow and sediment flux characteristics of the 20 largest streams entering the pacific ocean along the central and southern california coast extending for 750 km from monterey bay to just south of the u s mexico border drainage basins ranged in area from 120 to 10 800 km2 with headwater elevations ranging from 460 to 3770 m annual streamflow ranged from 0 to a maximum of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 1 times 10 9 end document m3 yr for the santa clara river in 1969 with a

subducted slabs and the geoid 1 numerical experiments with temperature dependent viscosity one of the most powerful constraints on mantle viscosity comes from the correlation of the long wavelength degree 4 9 geoid with that predicted by a density model for subducted slabs the effect of lateral variations of viscosity should be most pronounced at subduction zones due to the strong effect of temperature on viscosity an idealized slab model with temperature dependent viscosity is considered with various lateral and vertical viscosity structures using a two dimensional finite element formulation the viscosity parameterization affects the amplitude of the long wavelength geoid anomaly but not the sign of the correlation between the geoid and density anomalies depth dependent viscosity models with a high viscosity lithospheric layer do not completely match the temperature dependent laterally varying results suggesting that the rheology of the slab does have an effect on the long wavelength surface topography and geoid however this affect is minor suggesting that the radial models of mantle viscosity inferred from surface observables are correct to first order in contrast the short wavelength features are dramatically affected by the rheology of the region surrounding the downwelling further study of the shorter wavelength geoid over subduction zones may provide better insight into subduction zone processes

effect of lateral viscosity variations in the top 300 km on the geoid and dynamic topography summary we have investigated the impact of lateral viscosity variations lvv in the top 300 km of the mantle on the long wavelength gravitational response of the earth in contrast to the previous studies we demonstrate that the lvv may play a crucial role if a model with imposed plate velocities and partial layering is considered assuming lvv in the top mantle associated with the continental roots and the oceanic asthenosphere we are able to explain a significant portion of the geoid 85 per cent and of the free air gravity data 55 per cent without invoking a complex radial viscosity profile or assuming a lithosphere with an unrealistically low uniform viscosity the best fit to the data is found for models in which mass anomalies located at the boundary between upper and lower mantles reduce the mass flux across the 660 km discontinuity by a factor of 3 these models are characterized by a low average value of viscosity in the asthenosphere below the oceans 10 19 pa s and a large viscosity contrast 100 between the young suboceanic mantle and the deep continental roots the viscosity in the deep upper mantle is found to be around 3 10 20 pa s while a value higher by at least two orders of magnitude is typical for the lower mantle the predicted dynamic surface topography is influenced by the surface plate velocities and its amplitude amounts to a few hundred metres besides investigating the viscosity structures that reflect the different thicknesses of the lithosphere in continental and oceanic regions we have searched for a large scale pattern of lateral viscosity variation in the top 300 km that yields the best fit to the geoid the pattern obtained by this inversion shows a good correlation with the ocean continent distribution and 90 per cent of the geoid and 67 per cent of the free air gravity can be explained

holocene sedimentary activity in a non terrestrially coupled submarine canyon cook strait canyon system new zealand abstract the cook strait canyon system located between the north and south islands of new zealand is a large 1800 km 2 multi branching shelf indenting canyon on an active subduction margin the canyon comes within 1 km of the coast but does not intercept fluvial or littoral sediment systems and is therefore defined as a non terrestrially coupled system sediment transport associated with a strong tidal stream and seafloor disturbance related to numerous high activity faults is known from previous studies little is known however about the rates of sedimentary activity in the canyon and the processes driving it a substantial dataset of em300 multibeam bathymetry gravity cores 3 5 khz seismic reflection profiles camera and video transects and current meter data have been collected across the region between 2002 and 2011 the canyon system therefore provides an excellent study area for understanding sediment transport in a non coupled submarine canyon system analysis of the data reveals a two staged sediment transport system where 1 oceanographic tidal processes mobilise sediment from the continental shelf and transport it to depocentres in the upper central canyons and 2 tectonic earthquake processes remobilise sediment that is transported through the lower canyon to the deep ocean tidal boundary layer currents within the canyon reach velocities up to 0 53 m s and are capable of mobilising fine sand in the central reach of the upper canyons the velocity is higher at the canyon rim and capable of mobilising coarse sand sediment depocentres resulting from this tidally forced sediment transport have a well formed geomorphology within the mid upper canyon arms of cook strait and nicholson canyons pseudo static stability modelling supported by sediment core analysis indicates that sediment accumulated in the upper canyons fails during seismic events approximately every 100 years the 100 year return period ground shaking level peak ground acceleration ignoring the effect of the water column above the seabed at this site is estimated to be 0 23 g fresh rock outcrops and bed scour in the lower canyon floor indicate that remobilised material is transported to the deep ocean the processes identified here are likely to be analogous to those occurring in many non coupled shelf indenting canyons on active margins globally and provide a framework within which the biological response to geomorphic processes in submarine canyons can be assessed

effects of plate and slab viscosities on the geoid abstract the effects of plate rheology strong plate interiors and weak plate margins and stiff subducted lithosphere slabs on the geoid and plate motions considered jointly are examined with three dimensional spherical models of mantle flow buoyancy forces are based on the internal distribution of subducted lithosphere estimated from the last 160 ma of subduction history while the ratio of the lower mantle upper mantle viscosity has a strong effect on the long wavelength geoid as has been shown before we find that plate rheology is also significant and that its inclusion yields a better geoid model while simultaneously reproducing basic features of observed plate motion slab viscosity can strongly affect the geoid depending on whether a slab is coupled to the surface in particular deep high viscosity slabs beneath the northern pacific that are disconnected from the surface as a result of subduction history produce significant long wavelength geoid highs that differ from the observation this suggests that slabs in the lower mantle may be not as stiff as predicted from a simple thermally activated rheology if the slab model is accurate

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

the indian ocean gravity low evidence for an isostatically uncompensated depression in the upper mantle the broad gravity low in the equatorial indian ocean south of sri lanka is the largest and most striking feature in the gravitational field of the earth the most negative long wavelength free air gravity anomalies are found there and the sea surface geoid lies more than 100 meters below the best fitting ellipsoid we propose a model of the lithosphere and upper mantle which accurately predicts the observed free air gravity and geoid elevation this model is consistent with bathymetry and sediment thickness data and suggests that the crust south of india currently floats as much as 600 meters lower than would be expected if the region were isostatically compensated this residual depression of the crust is apparently confirmed by observations of ocean depth an uncompensated depression is consistent with the downgoing arm of a convection cell and also with the presence of a mechanical wake left in the upper mantle behind india as it traveled toward asia

solibore induced sediment resuspension in the faeroe shetland channel 1 high resolution measurements of temperature and current velocity over the seabed in the faeroe shetland channel fsc reveal an intense internal wave train referred to as a solibore propagating up the continental slope in addition to enhancing turbulent mixing a rotor formed at the leading edge of the solibore generates sediment fluxes o 102 greater than background levels the interaction of solibores which appear to evolve from baroclinic bores with the seabed and the resulting rotor provides a dominant mechanism for the upward transport of sediment over continental slopes that may counteract downward avalanching of material by gravity

the effects of 3d topography on controlled source audio frequency magnetotelluric responses we are investigating the 3d topographic effects on controlled source audio frequency magnetotelluric data two 3d topographic models are considered a trapezoidal hill model and a trapezoidal valley model different responses are generated including the amplitude of electric field amplitude of magnetic field apparent resistivity and phase data the responses distorted by the 3d topography are simulated for the source located both next to and on the hill valley our study indicates that all the electric field magnetic field apparent resistivity and phase data are influenced by 3d topography but to different extents these topographic effects depend on the transmission receiver topography geometry the transmission frequency earth resistivity and the roughness of the surface the effects in the near field generated by topography in the survey area are quite different from those in the far field because of the existence of the source compared to those in the far field zone the magnetic field and pha

slabs in the lower mantle results of dynamic modelling compared with tomographic images and the geoid abstract the recent increase in resolution of tomographic images of the earth s interior has enabled us to see slabs in the lower mantle on the other hand the distribution of slabs can be inferred from subduction history using a dynamical model of mantle flow comparison of tomographic images with model results can help to distinguish between alternative models of subduction history tomographic models and mantle flow models and thus improve our understanding of the earth here a simple dynamic model of subduction is used and some results are presented amounts and locations of subduction are inferred from published models of plate motion and plate boundaries for the past 120 ma the latter have been interpolated on 2 ma time intervals mantle flow driven by the density anomalies corresponding to subducted slabs is calculated with the method of hager and o connell for a viscosity structure that features an increase in viscosity from 4 10 20 pa below the lithosphere to 4 10 22 pa in the lower part of the lower mantle slabs are advected in the flow field in the model slabs sink on average about 1700 km through the mantle during 100 ma however the calculated average horizontal motion of slabs throughout most of the mantle is only about 600 700 km for slabs that have sunk to the lowermost mantle it is about 1000 km this is in accordance with the previously observed good correlation between subduction locations and lowermost mantle heterogeneities slabs sink fastest in regions with abundant subduction and little trench migration e g below east asia in the lowermost mantle predicted slab locations tend to lie in areas with high seismic velocities however there are large areas of fast seismic velocities where no slabs are predicted and which therefore presumably correspond to earlier subduction in some regions e g below japan our model approximately reproduces the observed shape of the slab in other regions where the agreement is poor the results can help to further constrain models of subduction history and associated mantle flow additionally the predicted geoid is computed and compared with the actual geoid predicted and observed geoid have similar magnitude and a number of features in common however an optimization of the fit is not attempted here since in order to do this also the hot upwellings from the lower boundary layer presumably in the form of plumes would have to be included

effects of lateral viscosity variations on the geoid 1 we investigate the effects of lateral viscosity variations lvvs on mantle circulation model predictions of the global geoid the present study is motivated in part by earlier findings that lvvs due to stiff slabs in the lower mantle have a strong influence on the geoid and that slabs in the lower mantle are perhaps no stronger than the ambient mantle however more recently it has been argued based on global seismic tomography models that lvvs are a minor effect on the geoid in the light of these seemingly contradictory results we re visit the problem of slab strength in the lower mantle we substantiate that the geoid calculated from tomography is hardly affected by the presence of lvvs whereas the geoid computed from global slab models yields a poor fit to the observed geoid when lvvs are considered however this degradation of fit only occurs for very long wavelengths of flow indicating inherent differences between the slab and tomography models we also investigate the effects on the geoid due to weak plate boundaries strong cratonic keels and a low viscosity region in the d layer due to post perovskite in addition to the geoid we attempt to fit plate motions with a circulation model that has prescribed weak zones at plate boundaries motions are matched well and taking into account lvvs the geoid with appropriate surface velocity boundary conditions agrees with the observed geoid as well as for free slip cases

in situ observations of floc settling velocities in glacier bay alaska abstract in situ floc settling velocities and diameters of particles ranging in size from 0 63 to 5 05 mm equivalent circular diameter were measured under a buoyant discharge plume by deploying a bottom tripod mounted floc camera assembly fca in tarr inlet glacier bay alaska these observations were used to estimate floc effective densities three results emerge from this work first fits of settling velocity and effective density to diameter are consistent with expressions published for other environments suggesting that common controls on floc size and settling velocity operate across diverse marine environments second the raw data show considerable scatter with upper and lower 95 prediction intervals on settling velocity and excess density differing by about a factor of 7 analysis of sources of error suggests that the variability is caused by differences in component grain composition among flocs and turbulent stirring within the stilling box third bin averaged effective densities and settling velocities are highly correlated with diameter thus while it is not possible based on diameter to predict accurately the settling velocity of a single floc it is possible to estimate the mean settling velocity of a population of like sized flocs

seismic evidence for slab graveyards atop the core mantle boundary beneath the indian ocean geoid low abstract the indian ocean geoid low iogl that spans a vast areal extent south of the indian subcontinent is a spectacular feature on the earth whose origin still remains ambiguous in this study we investigate the seismic character of the lower mantle below this geoid low utilizing the travel time and amplitude residuals of high quality s and scs phases from 207 earthquakes recorded at 276 stations in the epicentral distance range of 36 90 for comparison we also perform a similar exercise for a region of geoid high in the vicinity results reveal large variations in the scs travel times indicating that the lowermost mantle beneath the iogl region is heterogeneous the scs s differential travel times are 3 s slower than those predicted by the iasp91 model primarily due to velocity increase in the lowermost mantle beneath the iogl region and 2 s higher than the iasp91 beneath the geoid high region due to velocity decrease in the lowermost mantle the largest negative residuals from manual method 7 72 s are concentrated below the iogl iterative matching of differential travel time residuals reveals that the maximum positive and negative residuals can be explained in terms of a reduction in shear velocity of 0 9 and an increase of 1 6 respectively in a 1000 km thick layer above the core mantle boundary further the scs s amplitude residuals beneath the iogl are positive implying high impedance contrast at the core mantle boundary owing to the presence of high velocity material we attribute these high velocities to the presence of dehydrated high density slab graveyards atop the core mantle boundary beneath the indian ocean release of water at the mid to upper mantle depths due to the dehydration of subducted slabs causing a reduction in density and velocity of the ambient mantle could be responsible for the geoid low

evidence for long lived subduction of an ancient tectonic plate beneath the southern indian ocean in this study ancient subducted tectonic plates have been observed in past seismic images of the mantle beneath north america and eurasia and it is likely that other ancient slab structures have remained largely hidden particularly in the seismic data limited regions beneath the vast oceans in the southern hemisphere here we present a new global tomographic image which shows a slab like structure beneath the southern indian ocean with coherency from the upper mantle to the core mantle boundary region a feature that has never been identified we postulate that the structure is an ancient tectonic plate that sank into the mantle along an extensive intraoceanic subduction zone that migrated southwestward across the ancient tethys ocean in the mesozoic era slab material still trapped in the transition zone is positioned near the edge of east gondwana at 140 ma suggesting that subduction terminated near the margin of the ancient continent prior to breakup and subsequent dispersal of its subcontinents

typhoon and earthquake enhanced concentration and inventory of dissolved and particulate trace metals along two submarine canyons off southwestern taiwan abstract the influence of typhoon and earthquake events on the distributions of dissolved and particulate trace metals al fe mn cd cu pb zn is evaluated along the gaoping canyon and fangliao canyon in a tectonically active margin off southwestern taiwan the fangliao canyon was mightily struck by a large pingtung earthquake ml 7 0 in winter when terrestrial inputs were insignificant this large earthquake clearly enhanced the concentration and inventory of total suspended matter tsm dissolved and particulate metals by 2 5 fold in fangliao canyon and nearby gaoping canyon as compared to those in normal winter conditions the gaoping canyon is typically inundated with massive amounts of terrestrial materials during summer typhoons from the mountainous gaoping river strong contrast of particle and metal distributions can be found between summer and winter and between typhoon and regular summer periods in the gaoping canyon which was apparently caused by various strengths of river inputs followed by sediment resuspension and lateral advection at intermediate and near bottom depths along the canyon the water column and canyon wide inventories of tsm dissolved and particulate metals increased up to 2 fold higher during the post typhoon period than during the rainy period in summer in the gaoping canyon after extreme events most metals tend to increase their release into the dissolved phase through the increase of tsm concentration in addition to huge river inputs during typhoon both typhoon and earthquake induce energetic sea conditions that cause sediment resuspension and enhance the concentration and inventory of particles and metals in submarine canyons

subducted slabs and lateral viscosity variations effects on the long wavelength geoid summary the characteristic broad local maxima exhibited by the long wavelength geoid over subduction zones are investigated with a numerical model of mantle flow in a spherical axisymmetric geometry a synthetic model of buoyancy driven subduction is used to test the effects on the geoid caused by the depth of penetration of the lithosphere into the mantle by the viscosity stratification and by lateral viscosity variations lvv in the lithosphere upper and lower mantle the presence of anomalous slab density in the lower mantle guarantees geoid amplitudes comparable with the observations favouring the picture of slabs that penetrate the transition zone and sink into the deep mantle the viscosity of the lower mantle controls the long wavelength geoid to the first order ensuring a clear positive signal when it is at least 30 times greater than the upper mantle viscosity the presence of lvv in the lithosphere in the form of weak plate margins helps to increase the contribution of the surface topography causing a pronounced reduction of the geoid localized lvv associated with the cold slab play a secondary role if they are in the upper mantle on the other hand highly viscous slabs in the lower mantle exert a large influence on the geoid they cause its amplitude to increase dramatically way beyond the values typically observed over subduction zones long wavelength flow becomes less vigorous as the slab viscosity increases deformation in the upper mantle becomes more localized and power is transferred to short wavelengths causing the long wavelength surface topography to diminish and the total geoid to increase slabs may be then weakened in the lower mantle or retain their high viscosity while other mechanisms act to lower the geoid it is shown that a phase change from perovskite to post perovskite above the core mantle boundary can cause the geoid to reduce significantly thereby helping to reconcile models and observations

suspended sediment transport on the continental shelf near davenport california abstract suspended sediment transport on the shelves off santa cruz and davenport california is studied using field measurements and bottom boundary layer modeling strong transport events mostly occur during storms in winter the volume of winter sediment transport is at least one order of magnitude greater than that of summer spring transport rock outcrops on the inner shelf

gypsum a joint tomographic model of mantle density and seismic wave speeds 1 gypsum is a 3 d model of mantle shear wave s speeds compressional wave p speeds and density the model is developed through simultaneous inversion of seismic body wave travel times p and s and geodynamic observations while using realistic mineral physics parameters linking wave speeds and density geodynamic observations include the global free air gravity field divergence of the tectonic plates dynamic topography of the free surface and the flow induced excess ellipticity of the core mantle boundary gypsum is built with the philosophy that heterogeneity that most closely resembles thermal variations is the simplest possible solution models of the density field from earth s free oscillations have provided great insight into the density configuration of the mantle but are limited to very long wavelength solutions alternatively scaling higher resolution seismic images to obtain density anomalies generates density fields that do not satisfy geodynamic observations the current study provides a 3 d density model for the mantle that directly satisfies geodynamic and seismic observations through a joint seismic geodynamic inversion process notable density field observations include high density piles at the base of superplume structures supporting the general results of past normal mode studies however we find that these features are more localized and have lower amplitude than past studies would suggest when we consider both fast and slow seismic anomalies in gypsum we find that p and s wave speeds are strongly correlated throughout the mantle however we find a low correlation of fast s wave zones in the deep mantle 1500 km depth with the corresponding p wave anomalies suggesting a systematic divergence from simplified thermal effects in ancient subducted slab anomalies the cratonic lithosphere and d regions are shown to have strong compositional signatures however we argue that temperature variations are the primary cause of p wave speed s wave speed and density anomalies throughout most of the mantle

how turbidity current frequency and character varies down a fjord delta system combining direct monitoring deposits and seismic data submarine turbidity currents are one of the most important processes for moving sediment across our planet they are hazardous to offshore infrastructure deposit petroleum reservoirs worldwide and may record tsunamigenic landslides however there are few studies that have monitored these submarine flows in action and even fewer studies that have combined direct monitoring with longer term records from core and seismic data of deposits this article provides one of the most complete studies yet of a turbidity current system the aim here is to understand what controls changes in flow frequency and character along the turbidite system the study area is a 12 km long delta fed fjord howe sound in british columbia canada over 100 often powerful up to 2 to 3 m sec 1 events occur each year in the highly active proximal channels which extend for 1 to 2 km from the delta lip about half of these events reach the lobes at the channel mouths however flow frequency decreases rapidly once these initially sand rich flows become unconfined and only one to five flows run out across the mid slope each year many of these sand rich channelized delta sourced flows therefore dissipated over a few hundred metres once unconfined rather than eroding and igniting upflow migrating bedforms indicate that supercritical flow dominated in the proximal channels and lobes and also across the unconfined mid slope these supercritical flows deposited thick sand beds in proximal channels and lobes but thinner and finer beds on the unconfined mid slope the distal flat basin records far larger volume and more hazardous events that have a recurrence interval of ca 100 years this study shows how sand rich delta fed flows dissipate rapidly once they become unconfined that supercritical flows dominate in both confined and unconfined settings and how a second type of more hazardous and much less frequent event is linked to a different scale of margin failure

morphodynamics in sediment starved inner shelf submarine canyons lower st lawrence estuary eastern canada abstract the contemporaneous activity and sedimentary processes in a series of inner shelf submarine canyons located in the lower st lawrence estuary were examined using high resolution multibeam bathymetry and backscatter data the presence of crescent shaped bedforms csbs displaced upslope at the bottom of the canyons between 2007 and 2012 indicates that they are currently active through the remobilization of sediment by gravity flows however the shelf and shores of the region are characterized by the absence of sediment our results indicate that gravity flows currently eroding the canyon floors do not transport new material downslope coming from the shelf but rather remobilize in situ deglacial sediments within the canyon thalweg we suggest that slope failures and internal tides waves are responsible for sediment remobilization in these canyons although their role in the upslope migrating csbs is unclear this paper provides evidence that sediment supply is not a prerequisite for the modern activity of inner shelf submarine canyons when processes such as slope failures and internal tides waves are frequent enough to remobilize in situ sediments

the modern geoid and ancient plate boundaries abstract the non hydrostatic geoid is dominated by three large anomalies an area of high gravity potential in the equatorial pacific another stretching from greenland through africa to the southwest indian ocean and a semi continuous low region passing from hudson s bay through siberia to india and on to antarctica none of these three high amplitude greater than 60 m and long wavelength anomalies corresponds to present day plate boundaries however if the modern geoid is plotted over the positions of continents and plate boundaries at 125 ma b p reconstructed relative to hotspots a strong correlation emerges the modern geoidal low corresponds in position to the areas of subduction surrounding the pacific 125 ma ago the geoidal high now centered on africa is entirely contained within ancient pangaea and the equatorial pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central pacific the most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions the correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past or that there is a lag of at least 100 ma in response of the lower mantle to upper mantle conditions

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

joint modeling of lithosphere and mantle dynamics evaluation of constraints from global tomography models with the advances in technology seismological theory and data acquisition a number of high resolution seismic tomography models have been published however discrepancies between tomography models often arise from different theoretical treatments of seismic wave propagation different inversion strategies and different data sets using a fixed velocity to density scaling and a fixed radial viscosity profile we compute global mantle flow models associated with the different tomography models and test the impact of these for explaining surface geophysical observations geoid dynamic topography stress and strain rates we use the joint modeling of lithosphere and mantle dynamics approach of ghosh and holt 2012 to compute the full lithosphere stresses except that we use hc for the mantle circulation model which accounts for the primary flow coupling features associated with density driven mantle flow our results show that the seismic tomography models of s40rts and saw642an provide a better match with surface observables on a global scale than other models tested both of these tomography models have important similarities including upwellings located in pacific eastern africa iceland and mid ocean ridges in the atlantic and indian ocean and downwelling flows mainly located beneath the andes the middle east and central and southeast asia

mantle upwellings above slab graveyards linked to the global geoid lows the global geoid is characterized by a semi continuous belt of lows that surround the pacific ocean including isolated minima in the indian ocean ross sea and northeast pacific and west atlantic oceans these geoid lows have been attributed to mesozoic subduction geodynamic models that include slab graveyards in the lower mantle as inferred from seismic topography or from plate reconstructions correctly predict the general trend of geoid minima however these models fail to accurately reproduce localized geoid lows in the indian ocean ross sea and northeast pacific ocean here we show that the geoid lows are correlated with high velocity anomalies near the base of the mantle and low velocity anomalies in the mid to upper mantle our mantle flow models reproduce the geoid minima if the mid to upper mantle upwellings are positioned above the inferred locations of ancient subducted slabs we find that the long wavelength trough in the geoid is linked to high density slab graveyards in the lower mantle whereas upwelling regions in the mantle above 1 000 km depth cause discrete lows within the larger trough we suggest that this mode of upwelling in the mid to upper mantle is caused by buoyant hydrated mantle that was created by processes around and above subducted slabs

cyclone induced hyperpycnal turbidity currents in a submarine canyon 1 density currents such as turbidity currents are major transport agents in various terrestrial lacustrine and marine environments worldwide however a gap exists between those who study the deposits by turbidity currents turbidite on a field scale and those who study turbidity currents using small scale laboratory experiments and theoretical numerical models we report two typhoon triggered hyperpycnal turbidity current events observed in a submarine canyon our findings verify turbidite sequences with the characteristics of suspended sediment carried by passing turbidity currents that displayed distinct waxing and waning phases our study also confirms the direct link between typhoon triggered hyperpycnal flows in a small mountainous river and turbidity currents in a nearby submarine canyon that transport sediment to the deep sea efficiently

effects on the long wavelength geoid anomaly of lateral viscosity variations caused by stiff subducting slabs weak plate margins and lower mantle rheology abstract instantaneous flow numerical calculations in a three dimensional spherical shell are employed to investigate the effects of lateral viscosity variations lvvs in the lithosphere and mantle on the long wavelength geoid anomaly the density anomaly model employed is a combination of seismic tomography and subducting slab models based on seismicity the global strain rate model is used to represent weak low viscosity plate margins in the lithosphere lvvs in the mantle are represented on the basis of the relation between seismic velocity and temperature i e temperature dependent rheology when highly viscous slabs in the upper mantle are considered the observed positive geoid anomaly over subduction zones can be accounted for only when the viscosity contrast between the reference upper mantle and the lower mantle is approximately 1 0 3 or lower and weak plate margins are imposed on the lithosphere lvvs in the lower mantle exert a large influence on the geoid pattern the calculated geoid anomalies over subduction zones exhibit generally positive patterns with quite high amplitudes compared with observations even when the low activation enthalpy of perovskite in the lower mantle is employed inferred weak slabs in the lower mantle may be explained in terms of recent mineral physics results highlighting the possibility of grain size reduction due to the postspinel phase transition

interpretation of the lithospheric structure beneath the indian ocean from gravity gradient data abstract global geoidal undulations are often used as the constraining parameter to model the mantle convection pattern while gravity data are typically applied to interpret the lithospheric structure in more recent studies especially after launching the gravity field and steady state ocean circulation explorer goce satellite gravity gradiometry mission gravity gradient measurements have also been used in various geoscience applications the main motivation of using gravity gradients is to provide a more conclusive information about a spatial localization of investigated phenomena in geophysical studies such implications are particularly related to a probably better detection of investigated lithospheric structures such as active tectonic margins in this study we address this aspect by comparing interpretational characteristics from gravity and gravity gradient information the bouguer and mantle gravity gradients are used for this purpose to interpret the lithospheric structure beneath the indian ocean as well as the adjacent continental lithospheric structures such as the east african rift system our results confirm that gravity gradient more closely reproduces some tectonic and volcanic features such as oceanic subductions volcanic island arcs and convergent continental tectonic margins similarly mid oceanic spreading ridges are localized slightly better in the gravity gradient map our findings also indicate that the interpretational quality cannot be improved by using directly observed gravity gradients the reason is that these interpretations require applying gravimetric forward modelling techniques in order to enhance a particular lithospheric structure

thermochemical structure and dynamics of the african superplume 1 we present a new three dimensional 3 d model of the thermochemical structure of the african superplume region obtained from simultaneous inversion of global seismic and convection related data convection related observations include the global free air gravity field tectonic plate motions dynamic surface topography and the excess ellipticity of the core mantle boundary a 3 d image of the chemically induced density perturbations provides direct evidence that intrinsically dense material is entrained within the superplume and concentrated into a rounded structure 1000 km above the core mantle boundary the thermally induced density perturbations are greater in magnitude than the chemically induced implying overall positive buoyancy throughout the superplume the observed morphology and density signatures are consistent with a thermochemical plume that has risen from a compositionally distinct pile at the base of the mantle and may be currently deforming under the influence of its intrinsic negative chemical buoyancy citation simmons n a a m forte and s p grand 2007 thermochemical structure and dynamics of the african superplume geophys res lett 34 l02301 doi 10 1029 2006gl028009

in situ measurements of velocity structure within turbidity currents 1 turbidity currents are thought to be the main mechanism to move 500 000 m3 of sediments annually from the head of the monterey submarine canyon to the deep sea fan indirect evidence has shown frequent occurrences of such turbidity currents in the canyon but the dynamic properties of the turbidity currents such as maximum speed duration and dimensions are still unknown here we present the first ever in situ measurements of velocity profiles of four turbidity currents whose maximum along canyon velocity reached 190 cm s two turbidity currents coincided with storms that produced the highest swells and the biggest stream flows during the year long deployment

audiomagnetotelluric soundings on white island volcano abstract audiomagnetotelluric soundings in the frequency range 100 1 hz have been made at three locations within the crater region of white island volcano new zealand exceptionally low apparent resistivities are prevalent despite the possibility of terrain effects and static shift and the three dimensional nature of the island the measured values appear to be indicative of the true electrical structure one dimensional modelling of the e polarisation responses has been used to obtain approximations to the electrical structure beneath each site the results suggest a variable depth to the acid hydrothermal system which is believed to underly the crater region with low resistivities coming closest to the surface near the active fumaroles of donald mound

a late wisconsin 32 10k cal a bp history of pluvials droughts and vegetation in the pacific south west united states lake elsinore ca continuous sub centennially resolved paleo terrestrial records are rare from arid environments such as the pacific south west united states here we present a multi decadal to centennial resolution sediment core lake elsinore ca to reconstruct late wisconsin pluvials droughts and vegetation in general the late wisconsin is characterized by a wetter and colder climate than during the holocene specifically conditions between 32 3 and 24 9k cal a bp are characterized by large amplitude hydrologic and ecologic variability highlighting this period is a 2000 year glacial mega drought 27 6 25 7k cal a bp during which the lake shallowed 3 2 4 5 m depth this period is approximately coeval with a lake manix regression and an increase in xeric vegetation in the san bernardino mountains baldwin lake the local last glacial maximum llgm is bracketed between 23 3 and 19 7k cal a bp a 3000 year interval characterized by reduced run off relative to the glacial colder conditions and vegetative stability maximum sustained wetness follows the llgm beginning at 19 7 and peaking by 14 4k cal a bp a two step decrease in runoff characterizes the lateglacial to holocene transition however the vegetation change is more complex particularly at the beginning of the younger dryas chronozone by 12 6 12 4k cal a bp the climate achieved near holocene conditions

role of ocean continent contrast and continental keels on plate motion net rotation of lithosphere and the geoid three dimensional spherical shell models of mantle convection have been formulated to investigate the effects of ocean continent contrast i e different viscosity for continental and oceanic lithospheres and continental keels on plate motion net rotation of lithosphere i e degree 1 toroidal plate motion and the geoid the models include relatively realistic plate rheology i e strong plate interiors and weak plate margins and continental keel structure based on seismic models the mantle flow in the models is driven by slab buoyancy the models demonstrate that the ocean continent contrast and continental keels have minor effects on the long wavelength geoid for degrees 2 5 however continental keels overriding subducted slabs for example in north america can produce large negative gravity anomalies at a regional scale this may have important implications to interpreting the gravity anomalies in north america our models show that when plates bounded by weak plate margins have the same thickness neither weak plate margins nor the ocean continent contrast efficiently excite net rotation of lithosphere although they excite toroidal motion at higher harmonic degrees however plate thickness variations like continental keels excite the net rotation in spite of their ability to excite the net rotation continental keels as thick as 300 km cannot provide the necessary coupling to the deep mantle to produce the observed net rotation of lithosphere and slow continental motion if there is a weak asthenosphere underlying them if the thermomechanic structure of north american continental upper mantle derived from seismic and heat flow studies is representative our models suggest that the temperature and pressure dependent mantle rheology may not produce sufficiently high viscosity below continental keels that is needed to explain the observed plate motion our study hints the necessity to include more realistic treatment of plate boundaries in future studies to assess possible effects of plate plate coupling on plate motion

scaling analysis of multipulsed turbidity current evolution with application to turbidite interpretation deposits of submarine turbidity currents turbidites commonly exhibit upward fining grain size profiles reflecting deposition under waning flow conditions however more complex grading patterns such as multiple cycles of inverse to normal grading are also seen and interpreted as recording deposition under cycles of waxing and waning flow such flows are termed multipulsed turbidity currents and their deposits pulsed or multipulsed turbidites pulsing may arise at flow initiation or following downstream flow combination prior work has shown that individual pulses within multipulsed flows are advected forward and merge such that complex longitudinal velocity profiles eventually become monotonically varying although transition length scales in natural settings could not be predicted here we detail the first high frequency spatial vertical streamwise and temporal measurements of flow velocity and density distribution in multipulsed gravity current experiments the data support both a process explanation of pulse merging and a phase space analysis of transition length scales in prototype systems the point of merging corresponds to the transition in any deposit from multipulsed to normally graded turbidites the scaling analysis is limited to quasi horizontal natural settings in which multipulsed flows are generated by sequences of relatively short sediment failures 10 km sequences of breaches or where pulsing arises from combination at confluences of single pulsed flows such flows may be responsible for the pulsing signatures seen in some distal turbidites 100 km from source

can depression of the core mantle interface cause coincident magsat and geoidal lows of the central indian ocean abstract according to hide undulations at the core mantle interface distort geopotential fields that can be observed in lower order harmonics hide and malin obtained a significant correlation between long wavelength gravity and non dipole magnetic fields when the latter is displaced in longitude by 160 the central indian ocean is characterised by the largest geoidal low which correlates well with the magsat component low without any longitudinal displacement an integrated interpretation of geoidal and magnetic anomalies suggests that the source of these anomalies should be the same and may originate at the depression of the core mantle boundary the shallow curie isotherm the matching of the long wavelength of the geoidal anomaly with the vertical magsat anomaly and the identical location of the centres of the anomalies provide an excellent confirmation of hide s hypothesis but contradicts earlier geoidal low interpretations of mass deficiency in the earth s crust and mantle

two dimensional topographic responses in magnetotellurics modeled using finite elements we simulate the magnetotelluric response to two dimensional earth topography using finite elements linear interpolation of the secondary field parallel to strike over triangular elements allows accurate modeling of inclined resistivity boundaries including topographic surfaces to avoid discontinuities in field derivatives or resistivity care must be taken that the nodal values of the field parallal to strike used to obtain the auxiliary secondary fields are kept within uniform earth media the nodal locations may be shifted but the derivatives still are evaluated at the field points of interest correct values may be returned at gentle breaks in slope as well as along straight surfaces the finite element program is verified by comparison with the analytic transverse magnetic response of a hemicylindrical depression and with rayleigh scattering and transmission surface results for transverse electric and transverse magnetic polarization agreement with the other methods generally is excellent with t

topographic effects on telluric field measurements using the schwartz christofel transformation and numerical integration the effect of a sloping topographic irregularity on the telluric field measurements in a sedimentary basin is estimated results show that in the vicinity of the topographic feature the distortion introduced would increase with the angle of inclination of the sloping feature it is noticed that for moderate inclinations 20 50 the telluric field measured near the topographic feature is within 10 of its undisturbed value for distance greater than 0 1h 0 3h from the topographic feature whereh is the thickness of the sedimentary column overlying the resistive basement suitable charts are prepared to aid as means to arrive at the estimates of errors for various angles of inclinations of such topographic feature and also to help in formulating approximate rules of thumb for selection of station sites in a field survey to minimize such topographic effects

present deep submarine canyons activity in the bay of biscay ne atlantic present sedimentation in three canyons of the bay of biscay audierne blackmud and capbreton is studied by the combined analysis of cores and current meter data collected over a 7 month period at the current meter mooring locations interface cores were collected to characterize the recent sedimentation processes in the two canyons located in the northern part of the bay of biscay audierne and blackmud there is no evidence of recent sedimentary deposits canyons are by passing or erosive areas in the southern part of the bay of biscay capbreton recent turbidite are deposited in the three canyons current meters recorded energetic currents with velocities showing alternating upslope and downslope motions and a period corresponding to the semi diurnal component m2 these currents are supposed to be related to deep internal tides the high speed of the current 1 m s in audierne and blackmud is consistent with the lack of preservation of recent sediments on the canyon floors in capbreton canyon the magnitude of currents is less and recent turbidites are preserved in addition to periodical current motion small magnitude gravity event corresponding to a low concentration turbulent surge or a high concentration nepheloid layer initiated during a storm was recorded during the mooring period these results suggest that deep sea canyons in the bay of biscay have behaviour at present varying between by passing or erosion areas and sediment trapping these examples suggest that sediments are moved up and down by low energy tide initiated hydrodynamic events during most of the time during higher magnitude short duration gravity events the sediments are transferred down canyon towards the deep sea

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

savani a variable resolution whole mantle model of anisotropic shear velocity variations based on multiple data sets we present a tomographic model of radially anisotropic shear velocity variations in the earth s mantle based on a new compilation of previously published data sets and a variable block parameterization adapted to local raypath density we employ ray theoretical sensitivity functions to relate surface wave and body wave data with radially anisotropic velocity perturbations our database includes surface wave phase delays from fundamental modes up to the sixth overtone measured at periods between 25 and 350 s as well as cross correlation traveltimes of major body wave phases before inversion we apply crustal corrections using the crustal model crust2 0 and we account for azimuthal anisotropy in the upper mantle using ray theoretical corrections based on a global model of azimuthal anisotropy while being well correlated with earlier models at long spatial wavelength our preferred solution savani additionally delineates a number of previously unidentified structures due to its improved resolution in areas of dense coverage this is because the density of the inverse grid ranges between 1 25 in well sampled and 5 in poorly sampled regions allowing us to resolve regional structure better than it is typically the case in global s wave tomography our model highlights i a distinct ocean continent anisotropic signature in the uppermost mantle ii an oceanic peak in above average 1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness and iii a long wavelength pattern of 1 associated with the large low shear velocity provinces in the lowermost mantle

viscosity jump in earth s mid mantle the viscosity structure of earth s deep mantle affects the thermal evolution of earth the ascent of mantle plumes settling of subducted oceanic lithosphere and the mixing of compositional heterogeneities in the mantle based on a reanalysis of the long wavelength nonhydrostatic geoid we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth we detect an increase in viscosity at 800 to 1200 kilometers depth far greater than the depth of the mineral phase transformations that define the mantle transition zone the viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation plume deflection and changes in large scale structure and offers a simple explanation of these phenomena

hyperpycnal plume derived fans in the santa barbara channel california 1 hyperpycnal gravity currents rapidly transport sediment across shore from rivers to the continental shelf and deep sea although these geophysical processes are important sediment dispersal mechanisms few distinct geomorphic features on the continental shelf can be attributed to hyperpycnal flows here we provide evidence of large depositional features derived from hyperpycnal plumes on the continental shelf of the northern santa barbara channel california from the combination of new sonar lidar and seismic reflection data these data reveal lobate fans directly offshore of the mouths of several watersheds known to produce hyperpycnal concentrations of suspended sediment the fans occur on an upwardly concave section of the shelf where slopes decrease from 0 04 to 0 01 and the location of these fans is consistent with wave and auto suspending sediment gravity current theories thus we provide the first documentation that the morphology of sediment deposits on the continental shelf can be dictated by river generated hyperpycnal flows

internal tides in monterey submarine canyon abstract the m2 internal tide in monterey submarine canyon is simulated using a modified version of the princeton ocean model most of the internal tide energy entering the canyon is generated to the south on sur slope and at the head of carmel canyon the internal tide is topographically steered around the large canyon meanders depth integrated baroclinic energy fluxes are up canyon and largest near the canyon axis up to 1 5 kw m 1 at the mouth of the upper canyon and increasing to over 4 kw m 1 around monterey and san gregorio meanders the up canyon energy flux is bottom intensified suggesting that topographic focusing occurs net along canyon energy flux decreases almost monotonically from 9 mw at the canyon mouth to 1 mw at gooseneck meander implying that high levels of internal tide dissipation occur the depth integrated energy flux across the 200 m isobath is order 10 w m 1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head where internal ti

potential role of event driven sediment transport on sediment accumulation in the cariaco basin venezuela abstract a sediment density flow was observed in the eastern cariaco basin during september 2008 evidence suggests that this flow was likely triggered by a magnitude 5 2 earthquake that occurred on august 11 2008 with an epicenter located at 10 51 n 64 17 w off the city of cumana venezuela elevated suspended sediments near the bottom were observed at the mouth of the manzanares canyon 90 g m 2 over a depth of 165 m and decreased to 11 g m 2 over a depth of 40 m 42 km away from the canyon s mouth at the cariaco ocean time series site the sediment flux associated with this single event was 10 of the total annual sediment flux that typically reaches the cariaco basin deep seafloor carbon to nitrogen ratios and isotope composition confirm that most of the organic matter transferred by the sediment flow was of continental origin c n ratios of 17 67 13c of 27 04 and 15n of 6 83 our observations contribute to the growing body of evidence that suggests that submarine canyons are rapid efficient sediment conduits of particles from shallow to deep waters and that they should be included in efforts to constrain estimates of sediment and terrestrially derived carbon transport from the continental shelves to the deep ocean

constraining central neo tethys ocean reconstructions with mantle convection models a striking feature of the indian ocean is a distinct geoid low south of india pointing to a regionally anomalous mantle density structure equally prominent are rapid plate convergence rate variations between india and se asia particularly in late cretaceous paleocene times both observations are linked to the central neo tethys ocean subduction history for which competing scenarios have been proposed here we evaluate three alternative reconstructions by assimilating their associated time dependent velocity fields in global high resolution geodynamic earth models allowing us to predict the resulting seismic mantle heterogeneity and geoid signal our analysis reveals that a geoid low similar to the one observed develops naturally when a long lived back arc basin south of eurasia s paleo margin is assumed a quantitative comparison to seismic tomography further supports this model in contrast reconstructions assuming a single northward dipping subduction zone along eurasia s margin or models incorporating a temporary southward dipping intra oceanic subduction zone cannot sufficiently reproduce geoid and seismic observations

joint modeling of lithosphere and mantle dynamics sensitivity to viscosities within the lithosphere asthenosphere transition zone and d layers abstract although mantle rheology is one of the most important properties of the earth how a radial mantle viscosity structure affects lithosphere dynamics is still poorly known particularly the role of the lithosphere asthenosphere transition zone and d layer viscosities using constraints from the geoid plate motions and strain rates within plate boundary zones we provide important new refinements to the radial viscosity profile within the key layers of the lithosphere asthenosphere transition zone and d layer we follow the approach of the joint modeling of lithosphere and mantle dynamics ghosh and holt 2012 ghosh et al 2013b 2019 wang et al 2015 to show how the viscosities within these key layers influence lithosphere dynamics we use the viscosity structure sh08 steinberger and holme 2008 as a starting model the density variations within the mantle are derived from the tomography models which based on prior modeling had provided a best fit to the surface observables wang et al 2015 our results show that narrow viscosity ranges of moderately strong lithosphere 2 6 5 6 1022 pa s and moderately weak transition zone 5 9 3 1020 pa s as well as slightly large ranges of moderately weak asthenosphere 5 34 1019 pa s and d layer 4 8 18 1020 pa s are necessary to match all the surface observables we also find that a very strong lithosphere 8 6 1022 pa s along with a weak asthenosphere

a strong topographic valley effect in amt and vlf r measurements summary the topographic effects of sharply embanked deep and narrow valleys cut in resistive rock are very strong on mt amt and vlf r soundings when the widths of the valley floors are of the same order as the skin depths the electromagnetic responses of such 2 d valley models have been calculated by means of a finite element numerical method for both e and h polarization field configurations the most striking effects occur in the h polarization geometry where the entire current flow must round the bottom of the valley in concentrated bundles thereby creating large electric fields and high apparent resistivities ph with e polarization there is a sharp reduction of both electric and magnetic fields at the centre of the valley but the reduction is stronger for the electric field so the apparent resistivity pe decreases below the matrix resistivity when the bottom of the valley contains a fill of conducting material like sediments or alluviums it is found that both ph and pe are dramatically reduced to values below even the fill resistivity these effects are studied for a variety of ratios between skin depth and width of valley floor

layered mantle convection a model for geoid and topography the long wavelength geoid and topography are dynamic effects of a convecting mantle the long wavelength geoid of the earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow however the relationship of long wavelength topography to mantle circulation has been a puzzling problem in geodynamics we show that the dynamic topography is mainly due to density variations in the upper mantle even after the effects of lithospheric cooling and crustal thickness variation are taken into account layered mantle convection with a shallow origin for surface dynamic topography is consistent with the spectrum small amplitude and pattern of the topography layered mantle convection with a barrier about 250 km deeper than the 670 km phase boundary provides a self consistent geodynamic model for the amplitude and pattern of both the long wavelength geoid and surface topography

on the links between a river s hyperpycnal plume and marine benthic nepheloid layer in the wake of a typhoon abstract in 2010 two moorings each configured with a ctd and an adcp one with an additional non sequential sediment trap nsst were deployed in the head region of the gaoping submarine canyon 3 days after the typhoon induced peaks of the runoff and suspended sediment concentration ssc of the gaoping river in southern taiwan our data show a demarcation between tidal and hyperpycnal regimes in the temperature salinity and flow fields the latter existed in the first 5 days out of the 18 day deployment as defined by higher water density due to high ssc several lines of evidence indicate the presence of the tail end of a hyperpycnal turbidity current htc including the retention of warm water near the canyon floor high ssc down canyon directed residual flow and its vertical structure and high terrestrial fraction larger than 70 of the organic particles carried by the flow the decreasing mass flux during the deployment is also an indication of a waning htc our findings also show that the vertical flow structure and the direction of the gravity driven down canyon htc were retarded by the instantaneous up canyon directed tidal oscillations in the submarine canyon

suspended particle movement in and around quinault submarine canyon abstract time and space dependent light transmission temperature salinity and velocity data obtained during two winter experiments in the vicinity of quinault submarine canyon are used to examine processes controlling subtidal cpd fluctuations in suspended particulate concentration in the region in particular local resuspension of particles by waves and currents horizontal advection and vertical advection data include time series from four to eight mooring locations as many as 16 current meters and 10 transmissometers on the shelf over the open slope and in the canyon during each experiment spatial distributions from five ctd transmission surveys and gravity wave information particle concentration in the bottom boundary layer at the shelf edge was dominated by resuspension events the mechanism for resuspension had a large inter annual variation in the first year wave induced resuspension dominated whereas in the second year current induced resuspension dominated the cut off depth for storm related resuspension was generally m subtidal advection across isobaths upwelling downwelling accounted for 20 of the variance in concentration fluctuations at the shelf edge in the bottom boundary layer horizontal advection of material suspended in the bottom nepheloid layer on the shelf resulted in the formation of shelf break depth intermediate nepheloid layers over the canyon in regions where the flow crosses isobaths the existence strength onshore offshore extent and vertical structure of such nepheloid layers at a given time in this asymmetrical canyon appear to be a function of the direction of flow over the outer shelf and over the upper slope in the canyon whether active resuspension is occurring over the shelf and the maximum resuspension depth and magnitude of the active resuspension event the frequency timing and magnitude of previous resuspension events spatial and temporal continuity magnitude and direction of flow over the open slope north and south of the canyon vertical settling and upwelling or downwelling related to the along isobath flow over the upper slope fluctuations in particle concentration below the shelf break depth over the canyon slope were controlled by vertical rather than horizontal advection vertical excursions by the water mass of 100 m occurred both as a result of isopycnal adjustment to the quasi geostrophic along isobath flow and as a result of semidiurnal tidal oscillations when an overhead nepheloid layer was present such water mass excursions produced concentration fluctuations at depths below the maximum depth of storm related local resuspension in open slope regions where strong inls intermediate depth nepheloid layers were absent the portion of concentration variance that could be explained by advective models with constant spatial gradients was almost negligible only one resuspension event was documented in the bottom boundary layer within the deep canyon bottom depth 1000 m suggesting that advection must account for the observed variance in particle concentration however the large scale gradients at these depths are sufficiently weak cf the smaller scale gradients or noise that advective control could only be demonstrated during the second winter experiment when velocities along the canyon axis were relatively large finally the data demonstrate that subtidal concentration fluctuations within 50 m of the canyon floor are correlated to those at the shelf edge this correlation reflects a correlation in the velocity field rather than a transfer of particles as might occur with a turbidity current

event driven sediment flux in hueneme and mugu submarine canyons southern california vertical sediment fluxes and their dominant controlling processes in hueneme and mugu submarine canyons off south central california were assessed using data from sediment traps and current meters on two moorings that were deployed for 6 months during the winter of 2007 the maxima of total particulate flux which reached as high as 300 g m 2 day in hueneme canyon were recorded during winter storm events when high waves and river floods often coincided during these winter storms wave induced resuspension of shelf sediment was a major source for the elevated sediment fluxes canyon rim morphology rather than physical proximity to an adjacent river mouth appeared to control the magnitude of sediment fluxes in these two submarine canyon systems episodic turbidity currents and internal bores enhanced sediment fluxes particularly in the lower sediment traps positioned 30 m above the canyon floor lower excess 210 pb activities measured in the sediment samples collected during periods of peak total particulate flux further substantiate that reworked shelf rather than newly introduced river borne sediments supply most of the material entering these canyons during storms published by elsevier b v

periodic resuspension in baltimore canyon by focusing of internal waves high concentrations of particulate matter are frequently reported in submarine canyons this study demonstrates that in baltimore submarine canyon elevated concentrations result from periodic resuspension events the sloping bottom and v shaped topography of submarine canyons could accelerate tidal flows in the canyon axis sufficiently to cause resuspension which occurs predominantly at tidal frequencies however current meter and transmissometer data show that resuspension events are often associated with a borelike intrusion of cold water moving upslope near the canyon head an event that is consistent with the focusing of internal wave energy toward the canyon head at tidal frequencies hydrographic conditions for the focusing of energy are most favorable in late winter and early spring when stratification in the canyon is diminished direct observations reveal that the most pronounced resuspension events occur during those seasons bottom stresses generated in the canyon are sufficient to resuspend sediment along the axis at least between 200 and 800 m with little resuspension occurring deeper along the axis on the canyon walls adjacent slope or shelf during a resuspension event the sediment laden water is forced upcanyon then sloshes back down the canyon until it reaches a layer of higher density water detaches from the canyon floor and moves seaward along a density surface this is a means of transferring sediment and associated pollutants intercepted at the canyon head to deeper water across the shelf slope front over geologic time frequent resuspension and seaward transport may aid in the headward erosion of submarine canyons or at least inhibit their infilling during interglacial times

sediment fluxes and varve formation in santa barbara basin offshore california results from a year long sediment trap study provide the first direct observations of seasonal variability in sediment fluxes to the sea floor in santa barbara basin offshore california and are used to evaluate varve formation in this basin sediment fluxes throughout the year are dominated by lithogenic material biogenic sediments primarily silica are of secondary importance we conclude that the combination of high lithogenic fluxes and low biogenic fluxes during the fall winter period results in the deposition of dark laminae the onset of upwelling and high surface productivity in the spring results in high biogenic silica fluxes that last into the summer the relative contribution of silica to the total sediment flux is highest during this spring summer period and we suggest that light laminae are formed at that time this is consistent with previous suggestions that sediment input is the primary factor controlling varve formation in this basin

vlf ground surveys a powerful tool for the study of shallow two dimensional structures the suitability of vlf ground surveys in the investigation of shallow two dimensional structures is analyzed for such structures the polar formalism is derived necessary in practice since the transmitters are generally not in the structural strike or profile a simple vertical dike is considered to demonstrate the striking anisotropy which can be expected over such a structure in particular the high apparent resistivity along the direction of a well conducting dike and the low resistivity across it the theory is then confronted with the practical example of an asymmetrical vertical dike resulting from a strike slip fault modelling of the survey results is very successful and yields good confirmation of the polar behaviour vlf ground surveys thus provide a quick and powerful tool for the study of geological accidents within about 100 m of the surface

the magnetotelluric phase over 2 d structures summary in 1985 fischer gave a simple explanation for the behaviour of the e polarization magnetotelluric phase over a 2 d dike this phase behaviour is a direct consequence of the rearrangement of the current flow in a 2 d structure as opposed to the flow in a simple uniform half space in spite of its usefulness it is shown that the simple phase rule is only qualitatively correct for b polarization it seemed at first that the simple phase rule would not remain valid however it is found that if the very different current distributions for e and b polarization are correctly interpreted the simple phase rule retains its validity qualitatively arguments are given for the very general validity of this rule at the surface of any 2 d structure even in the presence of complicated topography the rule is best formulated in terms of two statements a the magnetotelluric phase is a continuous function over any structure even across outcropping resistivity contrasts and b where the current is drawn to greater depths the phase will rise and where the current is concentrated near the surface the phase will drop

the geological setting of santa monica and san pedro basins california continental borderland the california continental borderland s present configuration dates from about 4 to 5 106 years before present bp and is the most recent of several configuration of the southern california margin that have evolved after the north america plate over rode the east pacific rise about 30 106 years ago the present morphology is a series of two to three northwest southeast trending rows of depressions separated by banks and insular ridges two inner basins santa monica and san pedro have been the site for the california basin study cabs funded by the us department of energy the santa monica and san pedro basins contain post miocene sediment thicknesses of about 2 5 and 1 5 km respectively during the holocene past 10 000 years about 10 12m have accumulated the sediment entered the basin by one or a combination of processes including particle infall mainly as bioaggregates from surface waters from nepheloid plumes surface mid depth and near bottom from turbidity currents mass movements and to a very minor degree direct precipitation in santa monica basin during the last century particle infall nepheloid plume transport have been the most common processes the former process has been dominant in thecentral basin at water depths from 900 945m where characteristic silt clay is found with a typical mean particle diameter of about 0 0006mm standard deviation 2 skewness near zero kurtosis is typically 2 platykurtic these values indicate broad near log normal particle size distributions the calcium carbonate content averages about 10 and organic carbon about 4 surficial sediment bulk densities are 1 1 1 2 and accumulation rates range from 16 30mg cm 2yr1 in this central fine deposit bottom water oxygen contents are at or below 0 1 ml 1 1 so that bioturbation is inhibited thus preserving the primary sedimentary stratification there appear to be annual varves but the laminae couplets are not simple wet dry season pairs their formation is related to longer period climatic events el nino cycle of 3 to 4 years is dominant and is also affected by aperiodic bottom water flushing events the anoxic zone of bottom water has expanded over the past 300 to 350 years from a samll area in the northeastern basin floor ca 120km2 at 300 years bp and now covers most of the basin floor below depths of about 850m 1290km2 this expansion may have been primarily relaed to ocean circulation rate changes following the little ice age but is also likely to have been strongly affected by subsequent major changes in the natural drainages post european colonization the bottom water oxygen contents must also reflect the introduction of oxidizable pollutants coincident with the shift to agricultural mission era and then to urban use late 19th century to present of the adjacent mainland such localized effects are also suggested by the contrasting very large increases in the area of low oxygen bottom water in santa monica basin relative to the minimal changes in the area of anoxia in santa barbara basin over the same period and the lack of such zone in san pedro basin in the san pedro basin the bottom waters have remained dysaerobic so its sediments are bioturbated and preserve little or no evidence of primary stratification dumping of oil drilling muds and cuttings during the couple of decades prior to 1970 has left a surficial deposit ranging from 1 to 15cm in thickness forming a bull s eye isopach pattern as seen in radiographs bottom water oxygen content is usually 0 2ml o21 1 bottom infauna and epifauna are sparse central basin floor sediments water depths 800m of san pedro basin have textural characteristics similar to those of santa monica basin with distal basin floor surficial sediments being deposited primarily by particle infall and nepheloid flow at the present time mean diameters 0 006mm standard deviation 2 skewness 0 and kurtosis 2 platykurtic calcium carbonate content averages 8 10 and organic carbon is about 4 an estimate of the contemporary sedimentation rate was determined for a single box core of 25 29mg cm 2 yr1 from the central basin near the present distributary margin it cannot be considered necessarily to be representative mixing is indicated by the data an important climatic factor is the alternation of multi year wet and dry periods the california basin study was conducted during an extended dry period of low river discharge this undoubtedly will have affected the conclusions of the study related to sediment water interactions and pollutant transport by suspended clays

a long lived indian ocean slab deep dip reversal induced by the african llsvp a slab like high seismic velocity anomaly referred as seis has been inferred beneath the central southern indian ocean in a recent tomographic inversion although subduction has previously been suggested regionally by surface observations the new inversion is consistent with a north dipping slab extending from the upper mantle to the core mantle boundary cmb we propose that seis anomaly originated from an oceanic plate in the paleo tethys that was consumed by a south dipping intra oceanic subduction zone during the triassic and jurassic periods seis challenges traditional concepts of the dynamics of slab descent by its relatively shallow depths and a present day polarity opposite to the geometry of subduction geodynamic models show the upwelling mantle flow exerted by a thermochemical pile can hold and stagnate the descending seis slab at shallow depths for more than 100 myr the spatial distribution of resistance from the upwelling mantle flow can reverse the slab dip producing a structure consistent with seismic inversions as well as with our proposed tectonic scenario and geology constraining the tethyan tectonic domain the results suggest that slabs can descend through the lower mantle at rates substantially lower than 1 cm yr and even reverse their polarity through interactions with background mantle flow

some remarks on the behavior of the magnetotelluric phase the magnetotelluric phase in the medium of incidence over a good conductor varies from its value at the conductor surface usually close to 45 sup 0 to the characteristic standing wave phaseshift of 90 sup 0 over a distance of a few conductor skin depths much shorter than the vacuum wavelength at the frequencies considered over the top of a graben or at the surface of a two layer structure the basement can be looked upon as the matrix if the graben or the overburden is a better conductor it will carry an extra current density in phase with the electric field thus giving rise to an additional magnetic field also in phase with the electric field this drives the argument of the surface impedance toward smaller values for a more resistive graben or overburden the current is depressed corresponding to a reduction of the surface magnetic field by an amount in phase with the electric field this effect drives the phase of the impedance to values above the characteristic 45 sup 0 of a uniform conductor

sediment transport processes at the head of halibut canyon eastern canada margin an interplay between internal tides and dense shelf water cascading abstract to investigate the processes by which sediment is transported through a submarine canyon incised in a glaciated margin the bottom boundary layer quadrapod ralph was deployed at 276 m depth in the west halibut canyon off newfoundland during winter 2008 2009 two main sediment transport processes were identified throughout the deployment firstly periodic increases of near bottom suspended sediment concentrations ssc were recorded associated with the up canyon propagation of the semidiurnal internal tidal bore along the canyon axis carrying fine sediment particles resuspended from deeper canyon regions the recorded ssc peaks lasting less than 1 h were observed sporadically and were linked to bottom intensified up canyon flows 40 cm s 1 concomitant with sharp drops in temperature secondly sediment transport was also observed during events of intensified down canyon current velocities that occurred during periods of sustained heat loss from surface waters but were not associated with large storm waves high resolution velocity profiles throughout the water column during these events revealed that the highest current speeds 1 m s 1 were centered several meters above the sea floor and corresponded to the region of maximum velocities of a gravity flow such flows had associated low ssc and cold water temperatures and are interpreted as dense shelf water cascading events channelized along the canyon axis sediment transport during these events was largely restricted to bedload and saltation producing winnowing of sands and fine sediments around larger gravel particles analysis of historical hydrographic data suggests that such gravity flows are not related to the formation of coastal dense waters advected towards the outer shelf that reached the canyon head rather the dense shelf waters appear to be generated around the outer shelf where convection during winter is able to reach the sea floor and generate a pool of near bottom dense water that cascades into the canyon during one or two tidal cycles a similar transport mechanism is likely to occur in other submarine canyons along the eastern canadian margin as well in other canyoned margins where winter convection can reach the shelf edge