seismic evidence for anomalous crustal structure beneath mesozoic fracture zones in the gambia basin eastern equatorial atlantic abstract five sonobuoy refraction profiles shot in the eastern equatorial atlantic reveal the presence of thin and heterogenous crust in a zone at least 200 km wide near the southern margin of the gambia basin oceanic layer 2 v p 4 1 4 4 km s is detected on all profiles on some sections it is underlain by crustal material with apparent seismic velocities between 5 7 km s and 7 0 km s on others layer 2 directly overlies a zone with apparent velocities characteristic of normal upper mantle velocities 8 km s the formation of anomalous crust is attributed to a modification of accretion processes resulting from a close spacing of fracture zones during a period of slow spreading in the early cretaceous

complex structure along a mesozoic sea floor spreading ridge birps deep seismic reflection cape verde abyssal plain summary as part of an intensive study of a small area of oceanic lithosphere the british institutions reflection profiling syndicate birps acquired closely spaced deepseismic reflection profiles over the early cretaceous crust of the cape verde abyssal plain off west africa the survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea floor spreading axis isochron profiles and orthogonal dip lines oriented in the original direction of spreading flow profiles a large capacity well tuned airgun source and very quiet shooting conditions ensured a high signal to noise ratio for deep reflection devising a strategy for mitigating contamination from wrap around multiples arriving from previous shots enabled us to use the minimum possible shot point interval 50 m allowed for collecting long 18 s records data processing was oriented towards a medium with low root mean square velocity steeply dipping structure and pervasive low apparent velocity noise from diffraction at the top of the igneous crust the contrast between the isochron and flow profiles is striking isochron profiles are typically highly reflective throughout the igneous crust consisting of bright bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted moho reflection these reflections do not offset intracrustal or top basement structure and thus are not interpreted as faults an igneous intrusive origin seems more likely flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust continuing down in places to the moho dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust the dominant dip of these reflections is to the west towards the spreading ridge axis reflections from the vicinity of the moho while well developed in some places are not particularly prominent across the survey area moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles on isochron profiles dipping reflections occasionally flatten out into and may merge with the moho reflection on flow profiles as dipping crustal reflections approach the moho reflection they are usually abruptly cut off by it without extending deeper this survey shows how oceanic crustal structure can vary rapidly over relatively small areas provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust and gives a conclusive observation of faults that penetrate the entire oceanic crust

the ocean study area tectonic history from magnetic anomaly data and seismic reflectivity the ocean experiment is a detailed geophysical study of a region of the cape verde basin a dense network of new magnetic and gravity profiles has enabled us to constrain the spreading rate history of the region and the location of fracture zones the main features on the gravity profiles are lineated perpendicular to the seafloor spreading magnetic anomaly lineations significant along axis variability in spreading history suggests that the mid atlantic ridge behaved as a series of loosely coupled segments within which spreading was fundamentally asymmetric such variability is associated with a minor jump in the ridge axis which changes the offset and expression of one of the fracture zones deep seismic reflection and refraction lines were oriented parallel and perpendicular to the magnetic lineations seismic reflections occur at all levels within the crust decreasing in amplitude and coherence below the level of the moho analysis of the subbasement reflectivity provides compelling evidence that at least two major sets of dipping structure are present and are imaged separately on the two perpendicular sets of seismic profiles dipping reflections on flow line dip profiles which are interpreted as faults due to their association with offsets in the basement surface appear to strike parallel to the paleoridge axis the majority of reflections that may be identified as faults dip toward the west and although basement topography suggests that east dipping faults are also present no reflections may be interpreted unambiguously as such east dipping reflections observed only in the middle to lower crust have a more obscure origin dipping reflections seen on isochron strike profiles show clear contrasts in strength lateral coherence depth and dip population a number of these strike parallel to flow lines comparing reflection and refraction data shows that both the layer 2 layer 3 boundary and the moho are marked by a change in the character of reflections and suggests that they may represent important structural as well as seismological boundaries within the oceanic lithosphere

a seismic reflection profile study of lithospheric flexure in the vicinity of the cape verde islands 1 seismic reflection profile data are used to determine the stratigraphic architecture of the flexural moat that flanks the cape verde islands the moat region is characterized by upward of 1 2 km of poorly to well stratified material the two lowermost units thicken from west to east and are attributed to sediment loading and flexure at the nearby west africa continental margin during early cretaceous to early miocene the two uppermost units in contrast thicken concentrically around the islands and are attributed to the infilling of a flexural moat that formed by volcanic loading since the early miocene flexure modeling shows that the thickness of the moat infill cannot be explained only by surface loading and requires that the downward flexure due to surface loads is opposed by an upward acting subsurface load the best fit combined surface and buried loading model is for an elastic thickness te of 29 km a load and infill density of 2700 kg m 3 and a ratio of surface to subsurface loads f of 0 2 these results are in accord with spectral studies of free air gravity anomaly and topography data based on the admittance technique the subsurface loads are spatially limited to the islands and their submarine flanks nevertheless they are associated with a broad regional uplift of up to 400 m the uplift is large enough to explain why the moat infill is generally tilted away from rather than toward the islands the uplift is too small however to account for the height of the swell upon which the cape verde islands are superimposed the origin of the cape verde swell is not known however a normal te and a modest heat flow anomaly suggest that the swell cannot be fully explained by uplift due to thermal reheating of the lithosphere by an underlying hot spot and that other deep seated mantle processes must be involved

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

the volcanic and geochemical development of sao nicolau cape verde islands constraints from field and 40ar 39ar evidence abstract we present 34 new age results from 40ar 39ar incremental heating analyses of groundmass separates from volcanic rocks from sao nicolau cape verde combining the age results with field observations we show that the volcanic activity that formed the island occurred in four separate stages 1 6 2 5 7 ma 2 4 7 2 6 ma 3 1 7 1 ma and 4 8 wt basic to ultrabasic ranging from nephelinites through basanites to picrobasalts evolved rocks range to phonolites during all four volcanic stages predominantly high mgo rocks were erupted with 8 19 wt mgo olivine microprobe data from eight highly porphyritic of these rocks had fo86 88 demonstrating that even crystal enriched rocks represent primitive melts and in general it can be concluded that mantle melts reached the surface after only little modification during much of the development of sao nicolau only during stage 2 magma chambers were established and highly developed magmas erupted at this time the major part of the island was formed the new results constrain the evolution of sao nicolau to have taken place during a later time interval than previously assumed with more ages of only around 50 ka the youngest volcanic rocks found on the island indicate that stage 4 may still be active the cyclicity of sao nicolau volcanism suggests that mantle plume material arrived under sao nicolau in batches separated by nonfertile material

paleomagnetic results from a reconnaissance study of santiago cape verde islands identification of cryptochron c2r 2r 1 abstract new paleomagnetic data from three lava sequences on santiago cape verde islands are presented the paleomagnetic data are coupled with ar 40 ar 39 age determinations allowing detailed correlations between the three profiles and the geomagnetic polarity time scale gpts the younger of the profiles the ribeira da barca profile recorded flows of normal polarity that correlate with the brunhes chron whereas the porto formosa profile of reverse polarity correlates with the lower matuyama chron the sao goncalo profile consists of both reverse and normal polarity flows that belong to the matuyama and gauss chron respectively virtual geomagnetic poles vgps from a sequence of flow units in the sao goncalo profile classify as transitional and we interpret it as a geomagnetic event sg i well defined ar 40 ar 39 ages bound the occurrence of event sg i to the interval 2 37 2 43 ma 2 which is synchronous with the timing of cryptochron c2r 2r 1 in the marine magnetic anomaly record this study therefore confirms the existence of cryptochron c2r 2r 1 and indicates that it was a global scale phenomenon the vgps indicate that the field was in a meta stable transitional state during cryptochron c2r 2r 1 and that it spent some time in or around the paleomagnetic transitional vgp cluster patch close to australia

alluvial fans on volcanic islands a morphometric perspective sao vicente cape verde abstract the distribution morphology and development of volcanic island alluvial fans are investigated using sao vicente from the cape verde volcanic chain east central atlantic ocean this is a tectonically and volcanically inactive dryland setting comprising an eroded plio pleistocene composite volcano overlying a submarine shield volcano we mapped 228 alluvial fans quantifying fan f and catchment c morphological properties area a gradient g relief r length l fans and catchments of varying size and gradient are distributed around the island coastal margins 6 and along interior valley sides 94 regression results i e ve ve slopes of best fit lines conformed to broader fan research but with weak correlations and coefficient exponent values outside or at the lower end of values reported from non volcanic settings analysis of fan area and gradient vs catchment area regression residuals revealed fans were dominated by small steep gradient 28 and large low gradient forms 39 fan building space is a fundamental control specifically linked to volcano structure but with interplay between rock strength base level and climate fans around the volcanic edifice flanks are uncommon due to steep flank slopes where flank margin fans occur they are typically small and steep forms due to marine erosion fans along flank incised valleys are common large low gradient fans are associated with a major volcanic rift aligned valley structure with extensive erosion into altered and weathered edifice core lithologies small steep gradient fans are associated with edifice flank valleys steeper and narrower valley forms reflect incision into stronger lithologies of the composite volcano flanks restricting fan building space fans located within the volcano central depression are amongst the largest and lowest gradient types building into the eroded and unconfined edifice core space coalescence apex toe incision toe erosion and catchment stream channel backfilling displayed by most fans suggest landscape erosion dominates during more arid periods e g interglacials with the dryland setting generating limited sediment and insufficient flood runoff for transport and fan deposition fan building likely occurs during wetter periods linked to quaternary climate variability quaternary eustatic variations are relevant for the volcano edifice margins where sea level changes control fan building space and erosion

anomalous seismic crustal structure of oceanic fracture zones summary the seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the north atlantic the crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3 anomalous crust is present in both large and small offset fracture zones since they are among the most common tectonic features in the ocean basins and are particularly closely spaced on slow spreading ridges fracture zones represent a major source of seismic crustal heterogeneity we interpret the anomalous crust as a thin intensely fractured faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that in places are extensively serpentinized the unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness culminating in the very thin crust of the fracture zones at either end of the ridge segment

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

mesozoic deep water and tertiary volcaniclastic deposition of maio cape verde islands implications for atlantic paleoenvironments and ocean island volcanism the oldest rocks exposed on maio in the cape verde islands eastern central atlantic are morb type ocean floor pillow basalts lava breccias and hyaloclastics generated at an inferred latest jurassic earliest cretaceous spreading axis possibly near a fossil fracture zone interlava sediments include distal terrigenous turbidites from the west african continental margin latest ocean floor volcanism was accompanied by precipitation of ferruginous oxide sediments from hydrothermal solutions comparable with modern spreading axes the overlying latest jurassic tithonian and lower cretaceous sediments are deep ocean nannoplankton radiolarian chalks coeval with the similar eastern atlantic white limestone and the tethyan maiolica facies by the mid cretaceous albian cenomanian maio had subsided below the calcite compensation depth with accumulation of the characteristic atlantic anoxic black shale facies thin interbeds of limestones and siltstones are pelagic calciturbidites and distal terrigenous turbidites lacking volcanic material after uplift the cretaceous pelagics are overlain apparently conformably without tilting by shallower marine to subaerial pyroclastics and epiclastics which include tuffs agglomerates conglomerates and limestones the existence not only of rounded water worn clasts of mesozoic ocean floor rocks but also of alkaline extrusive and intrusive igneous rocks points to derivation from an early plutonic complex of unknown age the earliest preserved extrusives which are for the most part fragmental basalts and trachytes are possible paleogene to miocene in age further dike and sill intrusion then occurred followed by westward directed folding and thrusting diapiric doming of the central igneous complex ensued followed by deep erosion and then eruption of now gently inclined ankaramites by late in the miocene fan deltas radiated from a rapidly eroding still volcanically active island the neogene saw more quiescent alkalic extrusion punctuated by peneplanation maio island is thus a clue to both mesozoic paleoenvironments in the eastern and central atlantic and to tertiary island building related to hot spot activity additional tabular matter for this article may be secured free of charge by requesting supplementary data 84 9 from the gsa documents secretary

an 40ar 39ar study of the cape verde hot spot temporal evolution in a semistationary plate environment the 40 ar 39 ar analyses of 28 groundmass separates from volcanic rocks from the islands of santiago sal and sao vicente cape verde archipelago are presented the new age data record the volcanic evolution for santiago from 4 6 to 0 7 ma for sal from around 15 to 1 1 ma and for sao vicente from 6 6 to 0 3 ma the major submarine constructional phase of santiago was erupted within a few hundred thousand years interval around 4 6 ma most of the subaerial santiago volcanic rocks were erupted in a second episode from 3 3 to 2 2 ma and late volcanism occurred at 1 1 0 7 ma volcanism on sal evolved in five stages 1 poorly constrained early miocene activity 2 16 14 ma 3 12 8 ma 4 around 5 4 ma and 5 1 1 0 6 ma sao vicente was constructed during three active periods 1 6 6 5 9 ma 2 4 7 4 5 ma and 3 0 3 ma sr isotope analyses of carbonates from maio confirm an early cretaceous age for limestones deposited on the seafloor and later uplifted the cape verde rise is indicated to have fully formed in the early miocene around 22 ma accompanied by the initial alkaline volcanism considerable volcanism on sal boa vista and maio took place in the miocene and pliocene and extended over much larger areas than the present islands whereas volcanism of the southwestern and northwestern island groups developed mainly during the pliocene and pleistocene and was mostly confined to the present island areas the periods of volcanic activity may be broadly correlated between the northwestern and southwestern groups of islands young volcanism 0 3 0 1 ma throughout the northwestern group extends along a 150 km long nw se trending lineament a relatively moderate average melting rate for the hot spot over the 22 ma period is estimated at 0 026 km 3 a corresponding to a total volume of 570 x 103 km 3 of magma emplaced in the crust and a mantle volume flux of 28 m 3 s much lower than iceland or hawaii the archipelago is situated to the south and sw of the center of the mantle plume anomaly and ahead of its relative movement the timing and location of volcanism suggest that mantle melting takes place in three channels an eastern one that has been active for 22 ma and in southwestern and northwestern channels since late miocene

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

laboratory determination of smear zone due to vertical drain installation this paper is mainly concerned with a laboratory study to investigate the effect of smear due to vertical drain installation the extent of the smear zone around a vertical drain was studied utilizing a large scale consolidometer apparatus the test results reveal that a significant reduction in the horizontal permeability takes place toward the central drain whereas the vertical permeability remains relatively unchanged the radius of the smear zone was estimated to be a factor of four to five times the radius of the central drain mandrel and the measured ratio of horizontal to vertical permeability approached unity at the drain soil interface the laboratory measured settlements are subsequently compared with the predictions based on the theory of hansbo and the finite element method it is of relevance to note that the inclusion of the correct variation of permeability ratios of the smear zone in the plane strain finite element analysis improves the accuracy of settlement predictions

the geological history of maio cape verde islands the oldest igneous rocks on maio are pillow lavas of mid ocean ridge pillow basalts character which have been tilted and uplifted about 4 km from the ocean floor to outcrop as a partial ring dipping steeply away from a central plutonic complex made up of pyroxenites essexites syenites and carbonatites the ocean floor volcanic rocks are overlain conformably by a stratigraphically continuous pelagic carbonate succession which demonstrates a shallowing depositional environment from the upper jurassic to upper cretaceous times when tuffaceous beds indicate renewed volcanism the tuffs are associated with rudites demonstrating the emergence of the island and amongst the clasts are plutonics indicating upper cretaceous magmatism and the unroofing of the volcano to a substantial depth deformation under compressive stress resulted in the folding and local repetition by thrusting of this sedimentary cover which together with the plutonic core had been intensively injected by major sills the mesozoic succession has been planed off and overlain with marked unconformity by a largely neogene sequence of volcanic and terrestrial sedimentary rocks there is a hiatus throughout the palaeogene and constructional activity appears to recommence with ankaramitic hyaloclastite and lava deltas and subaerial ankaramitic flows these are overlain by fluvial sediments and tuffs stratigraphically above these is an extensive plateau of silica undersaturated lavas olivine melilitites and nephelinites which rest on a planed and locally lateritized surface at topographically higher levels in the eastern part of the island there are thick ankaramitic lavas and pyroclasts which evidently flowed eastward through valleys cut down into the mesozoic strata and appear to be of pliocene age the subsequent history of the island appears to be non volcanic

vertical movements of ocean island volcanoes insights from a stationary plate environment abstract uplift reconstructions based on the cape verde s geological record provide a unique opportunity to study the long term isostatic movements associated with hotspot activity on a stationary plate environment the archipelago is considered stationary with respect to its melting source so the hotspot driven isostatic effects affecting the ocean islands are expected to be enhanced in this study ar ar and u th geochronology techniques were used to date a set of palaeo markers of sea level from santiago s and sao nicolau s edifices two of the main cape verde islands a comparison between relative sea level and eustatic sea level from a modern eustatic curve was established to extract the vertical displacement undergone by the markers and to reconstruct the uplift subsidence history of each island the resulting uplift reconstructions confirm that both santiago and sao nicolau experienced a general uplift trend over the last 6 ma seemingly synchronous with the vigorous volcanic activity that built their exposed edifices these islands however exhibit different uplift histories despite their common uplift trend several uplift mechanisms were tested and a local rather than regional mechanism is proposed as the main cause of uplift generally unrelated with far field effects of surface loading this mechanism is probably associated with magmatic additions at crustal level

characterization of a smear zone around vertical drains by large scale laboratory tests installation of prefabricated vertical drains using a mandrel causes disturbance of clay surrounding the drain resulting in a smear zone of reduced permeability in this paper an attempt is made to characterize the smear zone using large scale laboratory model tests two tests simulating the cases of no smear and with smear were conducted excess pore water pressures were monitored at seven different locations along the radial direction in addition undisturbed samples were collected at various locations in the clay layer for conducting oedometer tests the distribution of excess pore pressure due to drain installation gave a clear indication of the extent of the smear zone the effect of reconsolidation on the properties of clay was found to be much greater than that of the remoulding of the clay the extent of the smear zone was also confirmed from the change in permeability of the clay layer in the smear zone obtained from oedometer tests the radius of the smear zone is about four times tha

sr nd isotopic evidence of terrigenous flux variations in the bay of bengal implications of monsoons during the last 34 000 years 1 sr and nd isotopic compositions of a sediment core from the bay of bengal are used to constrain the provenance and transport pathways of their terrigenous component during the 3 to 34 14c kyr bp the results clearly demonstrate a distinctly different source for detrital fluxes on the ninetyeast ridge from that of distal and western fan sediments there is a systematic pattern of climate related variability in 87sr 86sr and nd o values at the studied location pulses of low 87sr 86sr higher radiogenic nd and high silt content occurring at 5 7 kyr intervals suggest significant changes in the detrital source increased supply of irrawaddy derived sediments appears to be the dominant source for these pulses our results support the hypothesis that rapid cold events of north atlantic heinrich events are characterized by the intensified northeast monsoon

vertical movements of ocean island volcanoes insights from a stationary plate the origin of hotspot swells is still controversial and their development is at present poorly constrained and strongly model dependent however since islands are built on top of these structures the role of swell development in island uplift subsidence should be assessed

shelf sedimentation off the ganges brahmaputra river system evidence for sediment bypassing to the bengal fan the nature of shelf sedimentation seaward of the world9s largest sediment dispersal system is examined by using sedimentological and geochronological techniques on a unique suite of sediment cores and grab samples sediments from the ganges brahmaputra river system are currently accumulating on the shelf in water depths of less than about 80 m forming a clinoform like deposit similar to subaqueous deltas found off other major river systems the highest sediment accumulation rates on the shelf occur near the head of the swatch of no ground a major submarine canyon that indents the shelf west of the present river mouths this observation together with textural data suggests that river sediments are transported seaward and westward and that the swatch of no ground is currently a major conduit for the transport of sediments from the bengal shelf

clay minerals and sr nd isotopic composition of the bay of bengal sediments implications for sediment provenance and climate control since 40 ka abstract grain size clay mineralogy 87sr 86sr end and ams14c analyses of deep sea sediments cored in the central bay of bengal are used to reconstruct the evolution of provenances and climate control since the last glacial period clay minerals 87sr 86sr ratios and end values indicate a mixture of sediment from the himalaya source indian peninsula and indo burman ranges irrawaddy river these analyses show that the himalaya mountains and indo burman ranges irrawaddy river are the main suppliers before the holocene period while the inputs of erosion materials from the indo gangetic floodplain and the indian peninsula increase during the holocene period accompanied by reduced inputs of the indo burman ranges and irrawaddy sediments the regional source sink process is significantly controlled by the climate warm and wet climate conditions and enhanced indian monsoon intensity during the holocene period are responsible for the increase of sediment input from the indo gangetic plain and the indian peninsula by improving erosion in the source area input from the floodplain to the river channel and the intensity of the southwest monsoon current in contrast due to the decrease of the northeast monsoon current during the holocene period sediment input from the irrawaddy and indo burman ranges are constrained deposition center transition between the submarine fan and shelf during the last glacial period and the holocene period occurs in response to the sea level change which controls the sedimentary mode and thus significantly influences the transportation and deposition processes in the bay of bengal bob

comment on a technique for estimation of suspended sediment concentration in very high turbid coastal waters an investigation from gulf of cambay india by d ramakrishnan r bharti and m das marine geology 346 2013 256 261

anomalous heat flow and geoid across the cape verde rise evidence for dynamic support from a thermal plume in the mantle summary we describe results from 200 heat flow measurements across the cape verde rise in the north atlantic they show that the heat flow through normal 125 myr old crust is 45 5 3 4mwm 2 close to the prediction for a lithospheric plate model heat flow increases towards the centre of the rise reaching a maximum of 16 4 mw m 2 above the normal oceanic value the concommitant geoid anomaly at the centre of the rise is 7 6 0 3 m and the depth anomaly is 1900 200 m the anomalous heat flow geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms lithospheric reheating models alone whether involving transient or sustained thinning of the lithosphere fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the cape verde rise axisymmetric convection models provide a better fit to the observed data and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle uplift caused by expansion of the solid lithosphere is of smaller importance the observed geoid bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2 4 times 1020 pa s and a coefficient of thermal expansion in the range 2 0 2 5 times 10 5 k 1 the effect of melting in the ascending plume is to buffer the heat flow anomalies in the case of the cape verde rise partial melting is minimal but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20 25 mw m 2

consolidation analysis of a stratified soil with vertical and horizontal drainage using the spectral method a novel use of the spectral method to determine excess pore water pressure during vertical consolidation of multi layered soil with time constant material properties is presented considering a unit cell with combined vertical and radial drainage equal strain conditions are assumed in the analysis the novel adoption of material properties that vary in a linear fashion with depth allows arbitrary distributions of properties to be modelled by incorporating surcharge and vacuum loading that vary with both depth and time a wide range of consolidation problems can be analysed the spectral method is a meshless approach producing a series solution to the consolidation problem based on matrix operations accuracy can be improved by increasing the number of terms used in the series solution the model is verified by the analysis of selected case studies characterised by analytical free strain consolidation with thin sand layers surcharge only laboratory test and embankment trial with surcharge and vacuum

relationship between hydrology and seasonal distribution of suspended sediments on the continental shelf of the bay of biscay abstract numerous oceanographic cruises with hydrology water sampling drift current measurements carried out since 1980 on the continental shelf of the bay of biscay together with available noaa avhrr infra red images form the basis of a proposed explanation for the processes responsible for the distribution of suspended sediments on the shelf the seasonal hydrographic structure of continental shelf waters is of paramount importance in sediment distribution in summer there is an horizontal stratification of water masses and suspended sediment distribution is closely related to the thermo haline structure a fresher water mass with less suspended material lies on a thicker and more turbid homogeneous layer during winter when sediment discharge from the rivers often reaches its annual maximum an oceanic thermo haline wedge occurs on the shelf at around 100 m as a result winter turbidity values on the outer continental shelf are low comparable to summer values and a permanent nepheloid layer is never observed the wedge which lasts for several months may act as a filter preventing transport to the slope high turbidities on the external shelf and the continental slope are only measured in spring when the thermo haline wedge disappears it seems possible that during winter time suspended materials brought by rivers are deposited in the grande vasiere the large mud patch it is postulated that the position of this mud patch is linked to the long term stable location of the thermo haline front that separates oceanic waters from the colder and less salty coastal waters

suspended sediment distribution and residual transport in the coastal ocean off the ganges brahmaputra river mouth abstract an examination of suspended sediment dispersal in the coastal ocean off the ganges brahmaputra river system using shipboard and satellite data reveal distinct patterns of sediment distribution and sediment motion which help explain the dynamics of the world s largest sediment dispersal system during the low discharge period suspended sediment concentration distribution is mainly a function of tidal energy and the sediments are primarily transported as a suspension population satellite and sea bed data indicate that transport of river sediments is largely restricted to an area landward of about the 20 isobath during the low discharge period a simplistic estimate of residual transport and its separation into tidal and non tidal components indicate that non tidal transport contributes about 47 of the total transport and has transport directions mostly southwestward during the northeast monsoon the tidal residual transport which contributes about 53 of the total transport however does not show any particular direction for most of the stations although some landward stations show shoreward transport although few water column measurements were made during the high discharge period data from this and previous studies suggest that the dominant transport direction is to the southwest for most of the year

crustal structure and origin of the cape verde rise the cape verde islands are located on a mid plate topographic swell and are thought to have formed above a deep mantle plume wide angle seismic data have been used to determine the crustal and uppermost mantle structure along a 440 km long transect of the archipelago modelling shows that normal oceanic crust 7 km in thickness exists between the islands and is gently flexed due to volcano loading there is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle the observed flexure and free air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m 3 the origin of the cape verde swell is poorly understood an elastic thickness of 30 km is expected for the 125 ma old oceanic lithosphere beneath the islands suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere the lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that instead the swell is supported by dynamic uplift associated with the underlying plume

fracture zones across the cape verde rise ne atlantic more accurate locations have been made for 13 fracture zones across the cape verde rise and four previously unknown fracture zones mapped evidence for their existence is from analysis of the m series of magnetic anomalies together with evidence from seismic reflection profiles where these data exist the cape verde islands themselves lie among a series of five fracture zones the continuity of magnetic lineations and fracture zones across the region of the cape verde islands indicates not only that the islands lie on oceanic crust but also that the original mesozoic seafloor features and magnetization persist and were not obliterated by the neogene reheating uplift and volcanism

an estimate of the effective elastic thickness of the cape verde rise application of the response function technique to the load of the cape verde archipelago on the cape verde rise suggests the effective elastic thickness of the rise is only 15 3 km this is thinner than would be expected for 130 ma crust from previous observations worldwide which suggest a value of 30 10 km this thinning is considered to be substantial evidence that the lithosphere of the cape verde rise has undergone thermal rejuvenation comparison of this study with previous work at hawaii suggests that reduction of the effective elastic thickness over swells with large depth anomalies is related to plate motion over the mantle heat source

tidal influence on suspended sediment distribution and dispersal in the northern andaman sea and gulf of martaban abstract surface and water column profiles of suspended matter collected during april may 2002 and satellite images were used to study factors influencing suspended sediment concentrations sscs and dispersal in the northern andaman sea and gulf of martaban one of the largest highly turbid areas of the world s oceans perennial high ssc in the gulf of martaban is due to a combination of factors including resuspension of sediments by strong tidal currents shallow bathymetry and seasonal sediment influx from rivers from satellite images it was observed that in the central portion of the gulf of martaban the turbidity front oscillates about 150 km in phase with spring neap tidal cycles and the area covered by the turbid zone ssc 15 mg l 1 increases from less than 15 000 km2 during neap tide to more than 45 000 km2 during spring tide the sediment discharged by the ayeyarwady river is transported mainly eastward along the coast into the gulf of martaban occasionally during the winter monsoon period sediment plumes are seen heading westward into the bay of bengal turbidity profiles show that bottom nepheloid layers are actively transporting some of the sediments into the deep andaman sea via the martaban canyon

sediment transport in the shelf canyon swatch of no ground bay of bengal the swatch of no ground is a shelf canyon that deeply incises the bengal shelf near the ganges brahmaputra river mouth cuts the foreset beds of the subaqueous river delta and acts as temporary depocenter between river mouth and bengal fan sedimentation rates in the swatch of no ground are highest near the canyon head at 50 cm a 1 decreasing to 15 cm a 1 in 600 m water depth the canyon deposits consist of intercalated fine silt clay and coarse silt sand grained deposits in seismic profiles small scale sedimentary structures and parallel bedded layers reveal that sediment in the canyon is mainly deposited from suspension during fair weather conditions tidal currents are the dominant mechanism that transports plumes of suspended river load towards the canyon forming fine grained silt clay layers during storm conditions sediment is resuspended on the inner shelf and subaqueous delta east of the canyon storm generated bottom currents transport the resuspended sediment alongshelf to the canyon where the particles are trapped and form graded coarse silt sand layers channels and gullies in the swatch of no ground indicate active gravity driven currents with an erosional character in the upper canyon and non depositional character below 450 m suggesting that persistent or ephemeral currents presently export sediment to the bengal fan numerous slumps and slides observed in the canyon show small scale acoustically transparent layers that indicate sediment liquefaction presumably initiated along the steep canyon margin these mudflows move downslope and halt where the canyon gradient decreases one widespread acoustically transparent layer with an age of approximately 140 160 years bp can be traced throughout much of the canyon and is probably the result of large scale sediment remobilization triggered by a catastrophic event like an earthquake this may represent decennial to centennial scale events that remove large quantities of sediment and prevent the canyon from rapid infilling

design procedure for vertical drains considering a linear variation of lateral permeability within the smear zone a system of vertical drains with surcharge preloading is an effective method for promoting radial drainage and accelerated soil consolidation this study presents a procedure for the design of vert

consolidation theory for a composite foundation considering radial and vertical flows within the column and the variation of soil permeability within the disturbed soil zone to remedy the contradiction between the equal strain assumption and the flow continuity assumption at a soil column interface the traditional flow continuity assumption was abandoned and the radial and vertical flows within a stone column were incorporated to consider the column consolidation and deformation in a coupled fashion moreover two possible variation patterns of the horizontal permeability coefficient of soil within the disturbed soil zone were included to reflect the detrimental influence on the surrounding soil due to column construction in addition a linearly changed total vertical stress along the column depth was assumed to achieve a more realistic stress distribution in practice by considering the above mentioned characteristics the governing equations for this type of consolidation problem were developed the solutions for the governing equations were subsequently derived by using a new initial condition obtained from the equilibrium condition and equal strain assumption on the

staged fine grained sediment supply from the himalayas to the bengal fan in response to climate change over the past 50 000 years abstract the bengal fan as the largest submarine fan in the world receives a large amount of sediments discharged from the himalayas through the ganges brahmaputra g b river system however previous studies of this system seldom focused on the role of channels which are widely developed in the bengal fan in sediment transportation over the last glacial cycle here we discuss the coupled sedimentary archive in two gravity cores along the active channel in the bengal fan our findings are based on grain size clay minerals and sr and nd isotope compositions of these two sediment cores end member modeling of grain size data reveals that the intermediate end member represents the flux of distal fluvial particles from the g b river system the fine end member denotes regional sediment supply of weathered volcanic materials especially from the eastern indian peninsula and the coarse end member probably reflects nearby terrigenous input aeolian input and or volcanic glass sediment provenance analysis based on clay minerals and sr and nd isotopes confirmed sediment supply from the g b river system which was characterized by high illite percentage high 87sr 86sr ratios and low end values especially during five stages 50 45 ka 42 37 ka 31 28 5 ka 24 20 ka and 14 9 ka during these stages heavy fine grained sediment supply from the g b river system was discharged into the study area under the influence of climate change rather than sea level fluctuation additionally emphasizing the importance of channels in the sediment transport process moreover we find a significant alteration of sediment sources at 26 ka which may be related to conversion of the deep water currents in the bay of bengal

an analytical solution for consolidation with vertical drains under multi ramp loading various analytical solutions have been proposed for a unit cell consolidation with a vertical drain under surcharge loading these solutions involve different assumptions to address various aspects of consolidation there is a lack of generalised solution for analysing consolidation of soil assisted by a vertical drain under various loading conditions this paper presents a simplified solution for consolidation under multi ramp loading generalised governing equations of equal strain consolidation are solved simultaneous radial and vertical flow conditions as well as the combined effects of drain resistance and smear are taken into account fully an increase in total stress due to multi ramp loading is reasonably modelled as a function of both time and depth an analytical solution to calculate excess pore water pressure at any arbitrary point in soil is derived by using the method of separation of variables the conventional definition of the degree of consolidation is given in terms of the dissipatio